ROM hacks, also known as hack games or even ROM stains , are video games that were edited or altered by fans. The hackers achieve it by altering the code of this Read-Only Memory of a printed video game to make something brand new.

The core string Pokémon game system is considered very flexible and lends itself well to adjustment, giving hackers a lot of creative freedom together with a sound, well-tested game engine. These variables, in addition to Pokémon's remarkable popularity throughout the world, has lead to the development of tens of thousands of different games spanning a number of generations, ranging from minor adjustments to additional new attributes and, increasingly more often, entirely new fan-made Pokémon games. Though the core-series is most certainly the most popular string to hack, there are also many fan-created Pokémon games that do not use these as premise, and rather are, as an instance, modified versions of the Pokémon Mystery Dungeon collection and Pokémon Conquest.

There is a distinction between ROM hacks and"bootleg games". These are real copies of printed games which are offered to customers, often flashed onto after-market game paks or sport titles and lacking the official Nintendo Seal. However, since ROM hacks are usually dispersed across the Internet at no cost, they are regularly marketed as bootleg games, typically against the hacker's wishes. The reliability of these bootlegs are often inferior, and has become such a problem that hackers have started to introduce warning screens in their games, even cautioning investors who when they paid for it, they may have been scammed.

Historical Pokémon ROM hacks back into the late 1990s with altered versions of Pokémon Red. These were often relatively minor adjustments pokemon sword rom like sprite or dialog changes, and could normally follow the stream of the original game. These ancient ROM hacks were notorious for being especially crude in nature, ranging from swapping Pokémon and individual sprites round to replacing the dialog with disgusting text.

This trend continued until the launch of Pokémon Brown by user Koolboyman at the early 2000s, among the most notable Pokémon hacks ever. This is the very first hack to completely re-invent the match, with a brand-new area and totally new plot. Later versions included an expanded Pokédex (like Pokémon from Generations II, III and IV), additional dungeons and extra plot tweaks. It's one of the very few finished hack games and has been successful enough to warrant a sequel (Pokémon Prism, that a Pokémon Crystal-based hack) plus also a Generation III variant (Pokémon rijonAdventures) according to Pokémon FireRed.

Since Brown,'' Pokémon hacking notoriety has expanded exponentially, with developers creating many software tools for desktop operating systems to help create maps, edit sprites, images and even creating scripting languages specifically for the Pokémon video engine. It has also inspired many hackers to understand to edit the games' assembly code directly (known as ASM hacking). A lot of hacks are released, often with FireRed as a foundation, including Pokémon ShinyGold, Pokémon Naranja along with the Pokémon Ruby Destiny series, merely to name a couple.

There are numerous websites devoted to Pokémon ROM hack creation. Notable examples include Skeetendo, PHO and PokéCommunity's ROM Hack section, the largest known English-language Pokémon ROM hacking network. A number of these websites hold award nominations and ceremonies to showcase a few of the greatest talents of their neighborhood.