MissingNo. (Japanese: けつばんKetsuban, lit. "missing number"), as it is displayed in-game due to the ten-character limit in Western Generation I games, is a dual-type Bird/Normalglitch Pokémon in Pokémon Red and Blue, and a dual-type Normal/randomly named glitch type (which often has '9' in it) glitch Pokémon in Pokémon Yellow. It is arguably the best known glitch Pokémon, closely followed by 'M (00), and it is the easiest glitch Pokémon to find in the localizations. It has five distinct forms, but the most frequent forms (the Red/Blue and Yellow normal forms) share 36 index numbers each.
In later generations, other glitch Pokémon are sometimes referred to as "a MissingNo.", such as ??????????, ?, and -----. Despite this, the name "MissingNo." is a misnomer in this case; they have little relation to the one found in Pokémon Red and Blue or Yellow.
MissingNo.'s cry is generally the game's equivalent of a "blank" cry: a Nidoran♂ (base 00)'s cry with a pitch of 0 and no echo. This is because the cries of these Pokémon are undefined in the data structure. A few MissingNo. have different cries (specifically 0x43, 0x45, 0x4F, 0x51, 0x5E, 0x5F, 0x7F, and 0xB5). The fact that these cries are valid and not garbage data (i.e. unrelated data read in the wrong way, as is the case with most of MissingNo.'s properties) supports the view that MissingNo. may have originally been a Pokémon that was deleted from the games. (See this section for further details).
The Pokédex entries of MissingNo. and all glitch Pokémon with a Pokédex number of 0 will only appear upon capture if the player has not encountered Cubone, but as with all glitch Pokédex entries, they normally remain inaccessible through the actual Pokédex. An exception to this rule is in Red and Green, where MissingNo. can be "registered" in the Pokédex if the player enables the 'seen' flag for #152 through select glitches and memory corruption. If MissingNo.'s Pokédex data is viewed, Rhydon's cry will be heard, followed by MissingNo.'s (blank) entry and a series of glitchy sounds.
No MissingNo. can evolve or learn any moves through leveling up.
MissingNo.'s Red and Blue normal form is a dual-type Bird/Normal glitch Pokémon. It has a sprite that is a strange block of glitched pixels in a backward-L shape. Due to the larger number of characters that can be used to make it appear, it is the most common form. Normal form is exclusive to Pokémon Red and Blue.
This form appears through the old man glitch if the character in the third, fifth, or seventh slot of the player's name is the end-name marker, G, H, J, M, S, T, :, ], a, b, c, m, o, p, or v.
This form appears through the Mew glitch with a Special stat of 31, 32, 50, 52, 56, 61, 62, 63, 67, 68, 69, 79, 80, 81, 86, 87, 94, 95, 115, 121, 122, 127, 134, 135, 137, 140, 146, 156, 159, 160, 161, 162, 172, 174, 175 or 181.
MissingNo.'s Yellow normal form is a dual-type Normal and randomly named glitch type (often containing '9') glitch Pokémon. This form uses the palette of Pikachu, and so appears as yellow and red glitched boxes. They also cause glitches that MissingNo. in Red and Blue do not. If caught, they will cause graphical glitches, possibly rendering the player invisible and making duplicates of the player, and all other on-screen characters walk in random locations on the screen after battle. These forms do not appear through the old man glitch, as it was removed in Yellow. It is likely that the game will freeze or get stuck when attempting to battle Yellow MissingNo.'s normal form due to an invalid sprite pointer; 0x0600. This is also what causes the walking characters effect.
This form appears through the Mew glitch with a Special stat of 31, 32, 56, 61, 62, 63, 67, 68, 69, 86, 87, 94, 95, 115, 121, 122, 127, 134, 135, 137, 140, 146, 156, 159, 160, 161, 162, 172, 174, 175 or 181.
Encountering this MissingNo. form is not safe, as it usually causes the game to freeze or get stuck.
Fossil and ghost MissingNo.
The Fossil and ghost MissingNo. (otherwise known as 'special MissingNo.') are three unique forms of MissingNo. that share their sprites with the Kabutops Fossil and Aerodactyl Fossil from the Pewter Museum of Science, as well as the ghosts from the Pokémon Tower. This is because the game actually uses their index numbers to get their front sprites. These Pokémon have no constant base stats, experience types or starting moves; instead they take this data from the last Pokémon in the party other than another special MissingNo. If an enemy Trainer sends out a special MissingNo. the aforementioned data is taken from the previous Pokémon they sent out (the user's Pokémon in battle does not count here).
The Fossil and ghost MissingNo. exist in Yellow and have the same sprites as in Red and Blue. They also work the same way. Since the sprites are valid, these MissingNo. can be encountered instead of Yellow's normal MissingNo. and will never freeze the game on the opponent's side.
MissingNo.'s Ghost form uses the sprite of the ghost found in Pokémon Tower if the player does not have the Silph Scope. It is index number 184 (hex:B8)
In the Japanese versions, this form is not actually a MissingNo. In these versions, rather than being named 'けつばん' (ketsuban) it is named "Ghost" (Japanese: ゴースト). This is unused, because all Pokémon Tower ghosts including the deceased Marowak use the hiragana ゆうれい ('yuurei') instead, which can mean the same thing but may specifically refer to the Japanese interpretation of ghosts in folklore.
This form appears through the old man glitch if y is the character in the third, fifth, or seventh slot of the player's chosen name.
MissingNo.'s appearance in-battle allows the item duplication glitch, which increases the number of items in the 6th Bag slot by 128 unless the number of items in the slot already exceeds 128.
Encountering MissingNo. has been known to interfere with the save game data in various ways, such as adversely affecting the Hall of Fame data. The Hall of Fame is corrupted because of its proximity to a buffer used to store decompressed sprite data, which is too small to properly hold MissingNo.'s garbage sprite data.
When a Trainer battles with MissingNo. in their party, both the Trainer sprite and their Pokémon's sprites will be scrambled, and all other sprites will be reversed. Viewing the stats screen of a non-glitch Pokémon will remove the effects.
In Pokémon Yellow, capturing MissingNo. will commonly cause multiple player sprites, and sometimes other NPC's, to walk around the screen. If the NPC is aligned to the grid, it is possible to talk to them. Doing so may display text from the area, or may cause glitch text. Collision with those NPC's is buggy, but the game does handle it.
In Pokémon Yellow, if MissingNo is captured at a level above 1, it will instantly downlevel to 1 upon gaining any experience, and will stay on level 1 unless given Rare Candy. After this happens, Missingno will have seemingly infinite health, and, when damaged, the third digit of its health would, instead of going down through numbers, scroll through letters, numbers, and glitch symbols, and its HP bar will wrap across the screen.
In Pokémon Yellow, when MissingNo is encountered, if the game doesn't crash, it will sometimes play a very long cry before the text "A wild MISSINGNO appeared!" is displayed.
MissingNo. can be traded from Red and Blue into Pokémon Yellow, but cannot be traded forward through the Time Capsule into Pokémon Gold, Silver, and Crystal. In Yellow, MissingNo. levels down to level 1 if it gains any experience, and cannot grow any higher due to a glitched experience type. In Yellow, the only way to keep using a MissingNo. at a different level in battle is to use a Game Link Cable to battle between two games.
MissingNo. as a substitute doll in Pokémon Stadium
When viewed in Pokémon Stadium, MissingNo. will appear as a substitute doll (as will various other glitch Pokémon), while in Pokémon Stadium 2, MissingNo. (and other glitch Pokémon) will show up as a Ditto with Transform. If the game is saved via Stadium 2, it will then become a Ditto permanently.
MissingNo. as deleted Pokémon theory and unique cries
The name "MissingNo." is used for the invalid Pokémon with index numbers in between valid Pokémon and is a shortened form of "missing number", derived from its Japanese name, けつばん (Ketsuban). The fact that Rhydon, the Pokémon with index number 01 was the first Pokémon ever made has lead to speculation that the index numbers of Pokémon in Generation I represent the order that they were programmed into the game, with MissingNo. being deleted entries.
Supporting this theory is the fact that of the 39 MissingNo., nine of them have unique cries that do not consist of all 00 values. These cries are located in a table in between real Pokémon cries, suggesting that they are not garbage data (unrelated data interpreted in the wrong way, as is the case with most of MissingNo.'s properties).
It is notable that the number 151+39 comes to 190; a round number that may have very well been the initial number of Generation I Pokémon considered. This is supported by a thread on Smogon where Zog says that Shigeki Morimoto; a Pokémon designer confirmed that there were 190 Pokémon, and that 'the rest were saved for later'.
Official Nintendo statement
Nintendo has an official description of MissingNo. listed in their Customer Service troubleshooting section:
To fix the scrambled graphics, try releasing the MissingNo Pokémon. If the problem persists, the only solution is to re-start your game. This means erasing your current game and starting a brand new one." 
Nintendo's statement is incorrect on the method for fixing graphical issues; releasing MissingNo. does not fix the graphics and the player certainly does not need to erase their game to perform any fixes. The correct way to fix the graphics is to simply view the Pokédex data or stat page of any non-glitch Pokémon (or any hybrid glitch Pokémon). The statement may have been deliberately worded to discourage players from performing the Item duplication glitch.
MissingNo. is the only glitch Pokémon in Generation I whose English name is entirely composed of typographical characters. Although other glitch Pokémon such as Trainer and C may appear to be entirely composed of typographical characters, they contain illegible characters which change depending on the current tileset loaded into memory.
In the Generation IV games, there is a sprite that contains text that reads 欠番, MissingNo.'s Japanese name in kanji. However, unlike the Generation I placeholder text "MissingNo.", it exists only to substitute blank tile data, not blank species data. This means that the game will display this if a sprite is referenced that does not exist or cannot be accessed.
MissingNo. is referred to as "Pokémon 000" on Nintendo's consumer support website, referencing its Pokédex number.
In August 2016, TCG artist TOKIYA put art of MissingNo.'s Ghost form, Aerodactyl and Kabutops fossil forms, and default sprite in Pokémon Red and Green on his Twitter; MissingNo. is therefore the only glitch Pokémon to have an intentional depiction by a franchise artist.
MissingNo.'s English and Japanese names are both derived from "missing number".