Commander Keen is a series of side-scrolling platform video games developed primarily by id Software. The series consists of six main episodes, a "lost" episode, and a final game; all but the final game were originally released for MS-DOS in 1990 and 1991, while the 2001 Commander Keen was released for the Game Boy Color. The series follows the eponymous Commander Keen, the secret identity of the eight-year-old genius Billy Blaze, as he defends the Earth and the galaxy from alien threats with his homemade spaceship, rayguns, and pogo stick. The first three episodes were developed by Ideas from the Deep, the precursor to id, and published by Apogee Software as the shareware title Commander Keen in Invasion of the Vorticons; the "lost" episode 3.5 Commander Keen in Keen Dreams was developed by id and published as a retail title by Softdisk; episodes four and five were released by Apogee as the shareware Commander Keen in Goodbye, Galaxy; and the simultaneously developed episode six was published in retail by FormGen as Commander Keen in Aliens Ate My Babysitter. Ten years later, a homage and sequel to the series was developed by David A. Palmer Productions and published by Activision as Commander Keen. Another game was announced in 2019 as under development by ZeniMax Online Studios, but was not released. Invasion of the Vorticons was the only game developed by Ideas from the Deep, and was based on programmer John Carmack's creation of adaptive tile refresh, a technique that allowed IBM-compatible general-purpose computers to replicate the smooth scrolling of video game consoles such as the Nintendo Entertainment System. The game's success caused designer Tom Hall, programmers John Carmack and John Romero, and artist Adrian Carmack to found id Software. Their obligations to Softdisk, where they had worked during development of the game, led to the creation of Keen Dreams as a prototype for the second trilogy of episodes. The final episode was split off during development into a stand-alone retail title, and plans for a third trilogy were cancelled after the success of Wolfenstein 3D (1992) and development focus on 3D first-person shooters such as Doom (1993). The final Keen game ten years later had oversight but little development work from id. Critical reception and the series' legacy has focused on the two main trilogies of episodes, with Vorticons having large success as a shareware game and impacting the success of