Crossover roleplaying goes back approximately as far as roleplaying itself does. The first roleplaying game, Dungeons and Dragons, was based on existing tropes of the fantasy genre, and it didn't take long at all for people to start creating their own versions of Conan or Aragorn and seeing what would happen if they met.

Crossovers between different genres took a little longer. One of the earliest examples was the very first 'What If Genre A and Genre B Collided' game. In an article entitled "Sturmesghutz and Sorcery", appearing in The Strategic Review #5, December 1975, D&D creator Gary Gygax outlined a scenario that featured a meeting between D&D orcs... and the WWII German Army.

However, these remained isolated examples, and mostly just what if ideas. The first serious attempt to profit from crossover roleplaying came in 1979, with the release of the Advanced Dungeons and Dragons Dungeon Master's Guide. Hidden away in its pages were suggestions on how to convert AD&D to two of TSR's other games: the post-nuclear sci-fi Gamma World and the Western Boot Hill. Admittedly, this was probably more driven by marketing than by any particular artistic desire, but even so, it marked a transition.

More crossovers followed, as did the first genre-blending games. The most well known of these was FASA's Shadowrun which combined fantasy races and magic with a cyberpunk world background in what would become one of the most successful game lines in roleplaying history.

Various other games were moving towards a different approach; the creation of a single universal system that could handle any genre - or combination of genres. The early superhero game Champions moved towards this one genre at a time, with the release of a series of sourcebooks that slowly expanded its range. Steve Jackson Games, meanwhile, was working towards the same goal from the opposite end - they created a single system first, then expanded into various genres. The system was GURPS, and its sheer versatility has kept it on the market ever since, with over 150 sourcebooks covering nearly every concievable genre - each of them inevitably containing a section entitled Crossover Campaigns, which detailed how to use the sourcebook in your hands with various other sourcebooks in the GURPS line.

Actual crossovers were rare outside of magazines. A few exceptions included GURPS Cthulhupunk, and the time-travelling Paranoia series of modules that crossed over with Cyberpunk and Twilight 2000 - and that would have crossed over with the Dr Who game if not for licensing difficulties. (The final game was eventually published as 'Dr Whom and the Paranoids of Alpha' in the "Vulture Warriors from Dimension X" supplement to Paranoia.

Other means of reaching the same goal followed. Palladium Games' Rifts setting was a genre-clash of epic proportions - that incidentally was fully compatible with all their previous games. West End Games ingenious TORG setting used a single set of rules of great flexibility for a world in which our Earth was invaded by alternate worlds representing other genres. White Wolf's World of Darkness setting featured (at last count) eight separate games which all took place in the same world, and all interacted with each other. The success of the World of Darkness rocketed White Wolf, a small start-up company, to the position of second largest behemoth in the gaming industry.

More universal systems followed, but few of them lasted very long. In terms of breadth, the only one really of note is the new d20 system - Dungeons and Dragons Third Edition. The unique provisions of the Open Games Licence opened up the system to various third party publishers, and a slew of material for all genres followed. Several other game lines were converted to work with d20, including such venerable systems as traveler and Call of Cthulhu. (Ironically, Traveler is no longer available as a seperate system, but only as supplements for both d20 and GURPS.) D20 is the most popular current system, although how many crossover games are being run in it is another question.