General

No two hitobjects on the same tick (osu!mania game mode is an exception). This includes hit circles, starts and ends of sliders, and starts and ends of spinners. Although this may be possible to accomplish while playing, it defeats the purpose of following the rhythm of the song.
Do not overuse kiai time. The general rule of thumb is one toggle per minute of playtime. Kiai is meant to accent chorus sections, so don't use it just for fountains unless you're doing this rarely. You can toggle kiai off and back on halfway through the chorus if it fits, but using it constantly every few beats/measures is just ridiculous and can distract players.
Do not manually edit anything in an .osu file that cannot be changed through the Editor. The only exceptions are .osu-specific storyboards, slider velocity multipliers and skin-related options such as SliderBorder and SliderTrackOverride. If non-standard slider velocity multipliers are used, they must be announced in the beatmap description during the modding process.
There must not be any obscene imagery in the background/storyboard/video content. This includes nudity, near-nudity, sexual references, violence, drug abuse, etc. Keep things suitable for all ages.
Approved Category is only for Marathon maps. Long maps with over 5 minutes of draining time fit the Approval category. Only then they are allowed to be single difficulty mapsets. If they are below 5 minutes of draining time, a full difficulty spread is needed and the map will have to be ranked instead.

Mapset

Mapsets must have at least two difficulties of the same game mode, one of which must be an Easy/Normal level. It is possible for Marathon difficulties to have only 1 difficulty, but it must be named Marathon for the BSS to allow the submission to pending.
The mapset must have a well-designed spread of difficulties, containing at least an Easy or a Normal difficulty. This is so that players of all levels of experience are able to enjoy maps of the songs they love.
The difficulties in the mapset must be in a consecutive order. Easy or Normal can be skipped if the gap in the star rating spread allows it. The order can be seen in the chart below. If your mapset has two difficulties, one of them cannot be an Insane or Expert. The lowest difficulty must be below 2.0 stars. The difficulty level of Taiko-specific and osu!mania-specific difficulties must also follow a well-designed spread and might contain a Hard and Insane only, if there are standard difficulties present. In CtB, the spread evaluation is upon the BNs discretion. The difficulty spread is determined by the map's star rating. A map falls under a certain difficulty when having a specific star rating:
Below 1.50: Easy
1.50-2.25: Normal
2.25-3.75: Hard
3.75-5.25: Insane
Above 5.25: Expert
A difficulty's name must indicate its level of difficulty, with the exception of the hardest level of difficulty in a set. The mapset's hardest difficulty may use an appropriate custom difficulty name, unrelated to a username. Mapsets may also use a complete set of custom difficulty names that clearly indicate their level of difficulty to the player. Marathon maps with a single difficulty may use free naming.
Each difficulty must be able to be played by a single player. Thus, Multiplayer TAG-specific difficulties are not allowed since multiple people are required to play them. However, you may add an external link to a TAG difficulty in your map thread.
A mapset cannot have just one Taiko or osu!mania difficulty. If you decide, for example, to include Taiko in your map, make sure you have at least 2 Taiko mode difficulties and that one of them is of an easier difficulty level if it doesn't have an osu! standard Easy level difficulty. However having only one CtB difficulty is fine.
Having just one Catch the Beat specific difficulty is allowed within an osu! standard mapset, if it is not an Extra level difficulty. If you wish to add a CtB-specific Extra level difficulty into a osu! standard mapset, you must at least add another CtB-specific Insane difficulty. osu! difficulties are converted correctly into Catch the Beat. However, in mapsets without osu! standard difficulties you must create a full spreaded CtB set, as Taiko and osu!mania difficulties can not be converted into Catch the Beat.
No guest mapper should have more difficulties in the mapset than the creator, all modes together. For collaboration mapsets, the uploader should take part in all collab difficulties. The person who should upload the map is the one who contributed most to it. Maps with an ambiguous set of difficulties such as collabs will be evaluated on a case-by-case basis to ensure that the submitting mapper is properly accounted for.

Metadata

Metadata must be consistent across all difficulties. This is to ensure that searching is consistent and clean across a mapset.
Eastern artists must be written in the proper 'surname firstname' format (e.g. Japanese/Chinese/Korean). For artists which have a preferred romanisation, use this one. If you're unsure of your artist's name, then check previously ranked maps, Google, Wikipedia, or ask a BN for help.
Songs with Japanese titles must use the Modified Hepburn method of romanisation in the "Romanised Title" field. As a non-unicode field, long vowels such as "おう" and "うう" should be romanised into "ou" and "uu" to avoid macrons. (Refer to this link for more information). Loan words should be expressed using romanisation from the original language.
Songs with Chinese titles must be romanised in this method: each character must be romanised into a single, capitalised, separated word. The official romanization should be used if provided. Loan words should be expressed using romanisation from the original language. Refer to Thread: Romanisation of Chinese for more information.
Vocaloids can be featured within the artist section if the producer/composer includes it via official sources. This ensures that we enforce metadata being accurate in all cases.
Do not alter the song's title. This includes adding any marker to describe the cut of the song such as "Short Ver.", "TV Size" and so forth unless those markers are part of the official song title as listed by a reputable source.
Only use the Source field if the song comes from or was made famous by a video game, movie, or series. Website names are not an acceptable Artist nor Source.
Tracks created by composers belonging to doujin circles should list the circle's name as the main composer. The sole exception to this is when a composer of a given circle is well-known enough by their own name. In this case, their own composer name may be used instead.
The information of multiple mapset contributors must be provided in the mapset, if there is any guest mapper. This might be in the creator's words, via a storyboard or via naming the guest difficulties appropriately. You only need to provide information of guest mapper and corresponding guest part. Guest mappers must be added to the tags of a mapset. This helps others to know if the map uploader is the main contributor of the mapset and who else contributed to the given mapset.

Timing

Uninherited (red) Timing Sections should be used to accurately map the song's timing. They should synchronize to the beats of the song as accurately as possible and use the correct time signature whenever possible. If an incorrect time signature would last for more than 2 bars, add another timing section to fix it. Please see this thread for more information on downbeats.
Your map must be perfectly timed. This means that your BPM and offset are spot-on, sliders end when they should, notes are generally following a recognizable rhythm (such as the lyrics or drums) which is comprehensible by a player, and that there are no unsnapped notes (you can check this by running AIMod (shortcut ctrl+shift+a) in the editor).
Uninherited timing sections must be the same in every difficulty of the mapset. That is, each section must have the same BPM and offset in each difficulty. Furthermore, there must not be extra or missing uninherited sections in any difficulty. A song's timing doesn't change between difficulties, so there's no sense in having different uninherited sections per difficulty.
Uninherited timing sections cannot be used to manipulate slider speeds. These can be very unexpected for the player while also disrupting the main menu pulse and slightly changing the overall timing of the map.
No two uninherited or two inherited timing sections should be placed at the same point. An inherited timing section may be placed on an uninherited timing section (but only to change the slider speed). Having two uninherited or two inherited sections on top of each other will cause problems within the beatmap.

Audio

A beatmap set may only contain one music file used by all difficulties. Multiple music files within a single beatmap set is unsupported by osu! and results in unexpected behaviour with preview times, metadata, etc.
The song's audio file must be of reasonable quality. Try and source mp3 files yourself; ripping them from a streaming video site often results in low quality audio with high file sizes. The bitrate of a beatmap's audio file must be no lower than 128kbps and no higher than 192kbps. If you are having trouble acquiring an appropriate audio file, contact one of the more audio-savvy BN; they will be more than happy to help find an mp3 for you.
The minimum draining (play) time for a map is 30 seconds (but the recommended minimum is 45 seconds). If your map is shorter than that, then try looping the song with an audio editing program. This is so each map gives a high enough score, and also so people do not try and cut songs too short for no reason, thus stopping people from enjoying the song fully.
You must cut your mp3 if you're using less than 80% of it. This doesn't count intro time, and thus only applies if more than 20% of the outro is unmapped. If you only plan to map a portion of your song, then including the full mp3 is a waste of file space. Many mp3 editing tools like this exist which make fading and cutting a song simple. However, if you have a legitimate reason for keeping your full mp3 (e.g. a storyboard after the map ends), then that is fine.
You must use hitsounds. Without these, things get way too monotonous as you are throwing away one of the main elements of variation present in mapping. You don't need to place them on every note (and are discouraged from doing so), but they must at least be frequently heard when playing.
Hitsounds must be in wav. Hitsounds in wav format are preferred, as mp3 files will not loop correctly and have a short delay, between 0ms and 20ms, but mp3 is allowed only for a special purpose for the beatmap, like the usage of "reverse cymbal" that have delay on the start. Note that using ogg is unrankable.
Hitsounds from notes and sliders must be audible. These provide feedback for the player, and having them silent in a rhythm game doesn't make much sense. If you don't like the default sounds, then find replacements rather than silencing notes. You can use hitsounds from the Custom Hitsound Library or easily find others online. Lowering the volume of a few notes to provide a dampened effect is usually fine, but complete silence is always unacceptable. The end of a spinner (or even the entire spinner) the sliding sound of a slider, and the end of a slider can be silent, but only do it if it makes sense. Finally, you cannot silence both slider ticks and slider slides together.
Do not use sliderslide, sliderwhistle, and spinnerspin hitsounds in a way that would replicate a hitsound on a circle, slider start, slider end, slider tick, or slider repeat. Sliderslide, sliderwhistle, and spinnerspin hitsounds are considered continuous hitsounds, meaning that their files play from start to end and loop as one continuous sound for the length of the held object. Do not alter any section of a held object to play a single hitsound.
Songs/Maps with mature lyrics/themes must be marked with a label such as "18+" in the map's thread. This game is for all ages, and so a warning is needed for younger audiences. There is currently no way to distinguish this outside of the map's thread, but this may change in the future.
Each difficulty of the mapset must have a preview point set (and be consistent in every difficulty). This is used for both the song selection menu and the online thumbnail preview.
Every .wav file must be at least 100ms long to prevent issues with soundcards. If you want a silent/blank hitsound, then you must use a 0-byte .wav file.
NOTE: Due to a technical issue having any files that are 0 bytes in size in the beatmap folder will cause files after the (first) 0-byte file to be missing. Please do NOT use any 0-byte files. The one linked above is 44KB in size and won't cause issues.
Hitsounds must have an acceptable range of delay under 5ms. Every hitsound file should start in time, preferrably at 0ms. This is to ensure that every map doesn't sound mistimed, and therefore could provide acceptably synchronized rhythm feedback to players.

Video

A beatmap set may not contain multiple background video files. Multiple videos within a single beatmap set is not supported by osz2 and will result in errors during upload and processing.
The video's dimensions must not exceed 1280x720. As well, upscaling lower resolution video to a higher resolution should be avoided. This ensures video files do not become excessively large or resource intensive.
The video's offset must be correct and consistent between difficulties so that it follows the music how it was originally meant to be.
The video's audio track must be removed from the video file. The audio track in video is not used in osu!, so removing it reduces that filesize of the beatmap.

Skinning

If you are using any elements created by another community member, ask permission beforehand. Respecting the work of others is paramount and most people will be delighted to have their work featured in your projects!
Gameplay elements must be visible. You cannot make any element that will impair the playability of the map invisible as it will make the map unintuitive or even impossible to play.
Skinned elements must be cropped cleanly so they don't look like they have jpeg or pixelated artifacts around them or half-cropped shadows. If you don't have a program like Photoshop, then there are many free alternatives like Paint.NET and GIMP that can be used instead.
Background images must be at a maximum of 1024x768 pixels for 4:3, 1366x768 pixels for 16:9. Although 1366x768 (16:9) is recommended, most background ratios that are 4:3 or 16:9 should also work reasonably well (for example, 800x600 or 852x480). Using an image with a different ratio is fine, but will result in letterboxing (black bars at the top/bottom of the screen) or upscaling (resized image to fill the whole screen).
Keep background imagery safe. Images should be on a level that can be displayed on all-audience TV, on public signage, and of nature that doesn't require censoring in any country. If you have any possible edge-case scenarios, please PM peppy on IRC with a link to the image in question; a list of images which are/aren't acceptable will be built up to set some visible guidelines.
You must have a background image on every difficulty of your map. If your map has a storyboard, then you can easily fade the background out, but one is still required for the song selection menu, online thumbnail icon, and for players not downloading with video (if the map has one).
Hitbursts must be clearly distinguishable from each other (e.g. 300/100). The best way to ensure this is to use a different color for each hitburst. Otherwise, the player won't know if they're hitting beats accurately.
When skinning gameplay elements, complete sets of elements must be skinned. For instance, if you skin hitcircles, you must include a hitcircle.png, hitcircleoverlay.png, approachcircle.png, and a slider border color (this can be done by adding the line "SliderBorder: 255,255,255" (using RGB values) under the [Colours] section of each .osu file). The same logic applies to hitbursts, cursors, numbers, and anything else of this nature. If you didn't intend to skin an element in the set, then using the templateskin's version is okay. Setting a SkinPreference is also acceptable (e.g. forcing the default skin).
Combobursts must be oriented for the left side of the screen. Combobursts are aligned to the bottom and left sides of the screen by default, and are just horizontally flipped for the right side. Thus, you only need to worry about making sure that the comboburst is cleanly cropped on the top and right sides of the image.
Do not make any essential gameplay elements larger than template skin dimensions (especially for combobursts). Larger elements may effect gameplay performance, and thus should be avoided. However, any elements that do not directly effect gameplay (e.g. pause-back, play-skip, etc.) may be slightly larger if a legitimate reason is given.
There must not be any unused files in the map's folder except for the map's .osb file (since they sometimes get added even if the map doesn't have a storyboard) and storyboard .thumb files (since they are automatically created in image directories). Unused files add extra file size which is unnecessary.

Storyboarding

Storyboard images must be at a reasonable filesize and dimension for their usage. For most uses (e.g. full-screen images), the maximum is 1366x768 pixels. For widescreen storyboards 854x480 is preferred as it matches the internal resolution of the SB editor. For 4:3 aspect ratio images 640x480 is preferred.
The maximum dimensions for tall or wide scrolling images are 640x1440 and 1920x480 respectively. This is for the sake of gameplay performance, image loading time, and filesize.
There must not be any unnecessary transparency around storyboarded images, so crop and resize them as much as possible. If there are particular reasons for apparently useless transparency, then please explain them. For transparent sprites, there should be at least one pixel of transparent border around them so that interpolation (e.g. a black-bordered image on a black background) works properly (although a shadow or glow around the image will fix this problem as well).
Maps that use repetitive strobes, pulsing images, or flashing colors in the storyboard must use the epilepsy warning. There is one built-into the editor, which will display a visual warning at the beginning of the map and also mark the map's online thumbnail with a warning icon. Also, it helps to make a note of the flashing/strobes/etc. in the map's thread. This warning is absolutely needed so that players with epilepsy can avoid any danger.

Guidelines

Guidelines are important and should be followed in most maps. However, they are NOT rules, so they may be broken in special cases. If you want to break a guideline, ask yourself this: Does what I'm about to do make sense? Is it more fun to play like this compared to sticking to the guidelines? If you answer yes to both these questions, then it is probably okay.

When your guideline-breaking map gets modded, try to be as reasonable as possible: Do your best to explain your point of view and be open to suggestions.

The difficulty spread should be linear and reasonable. Linear difficulty spread means your difficulties have a comparable gap in star rating between each other. A reasonable spread means the usage of difficulty appropriate gameplay elements. If your mapset does not have an Easy difficulty your Normal difficulty should follow the general guidelines for Easy difficulties. If your Insane difficulty is at or above a 5 star rating it is recommended to include another Insane level difficulty between Hard and the 5+ Insane.
A maximum of three slider velocities should be used (including 1x). For example, you could have a single map using 0.6x, 0.8x, and 1x; or 0.75x, 1x, and 1.5x; etc. If more than three slider velocities are used, then they should make sense and be intuitive. If slider velocity changes are able to be merged (e.g. close values like 0.8x and 0.7x) while still flowing/working correctly, then they should be.
When including a slider velocity change, there should be a discernible change in the map's tempo. A spacing change, a short break in the map, or a slider containing at least one tick will help show the transition between them.
Make sure that you can pass each difficulty in your mapset. Continually test-playing your map is one of the best ways to spot mistakes and correct issues.
Your difficulties should all end at the same spot. Having a fully-mapped Normal/Hard and a half-mapped Easy just looks sloppy/lazy. A full Easy may look boring to you, but not to a player that can't handle the harder difficulties.
Kiai should be consistent throughout difficulties of your mapset (especially since it is shown on the main menu). If you have a guest difficulty from another mapper that uses different kiai, then it's fine as long as it still makes sense.
Kiai should start on a white tick (or more commonly, the big white tick called the downbeat) of a measure. Generally, the main part of the chorus will start at this point.
The song should not be too long. Aim for 3 minutes maximum; anything longer gets tiring. If you need help editing a song down to length, feel free to ask in the Beatmap General Questions forum.
Use breaks where possible. Even if they are only 5-10 seconds, it allows a bit of recovery and hand repositioning for players. However, try to avoid longer breaks that are in excess of 15 seconds. Breaks should especially be used with Easy/Normal difficulties.
Options such as letterboxing, countdown, and audio lead-in should be consistent between difficulties. Having cohesive settings throughout your mapset will look much more professional, although there are always scenarios where this is impossible. One difficulty might start at the beginning of the mp3, where a countdown is impossible but an audio lead-in is absolutely necessary; while another difficulty may start a few seconds in, where a countdown is usable but an audio lead-in is senseless.
Try to keep osz files under 10MB, or 30MB if you include a video/storyboard. Videos and storyboards can generally be decreased in size by lowering the quality of the video or trimming/resizing storyboard elements.
Avoid using storyboarded hitsounds. If a player misses the hitobject that the storyboarded hitsound lands on, then the hitsound will be played (which doesn't make much sense when this is a rhythm game utilizing audio feedback). Also, storyboarded hitsounds at places where there are no objects can confuse the player into thinking there was some unseen hitobject, which doesn't make sense either.
Slider tick hitsounds are discouraged. If you want to use them, then make sure that their volume is balanced (i.e. notably quieter than regular hitsounds). A very loud slider tick, especially when only used once or twice, would be extremely jarring.
Avoid going over a 5.0x storyboard load to help prevent lag on older computers. Resizing some of your storyboard images may help with this.