“What is the Avatar” is part genre study, part theoretical discussion of fictionality and embodiment in gaming. Its topic is what I call “avatar-based” singleplayer games, with particular emphasis on the kind of navigable real-time 3D graphical environments that were spearheaded by Doom, Mario 64 and To mb R a i d e r in the mid-nineties. The avatar is that which extends our embodied self into game space in a direct and intuitive sense, beyond merely agency or perspective, in a way that makes us belong to and exposed to a gameworld. I describe and discuss the characteristic features of avatar-based play in relation to games, fiction, computer technology and cinema, and define it against other forms and genres of computer gaming. In some ways the study looks more like a book than a doctoral dissertation. It follows a thematic rather than scientific structure, draws on an eclectic mix of theoretical traditions and concepts, and mixes theoretical presentation, analysis and discussion throughout.