Abstract. Understanding player distributions, sessions, and movements in a Massively Multiplayer Online Role-Playing Game (MMORPG) is essential for research in scalable architectures for these systems. We present the first detailed measurement study and the first models of the virtual populations in two popular MMORPGs, World of WarcraftTM and Warhammer OnlineTM . Our results show that while these two types of MMORPGs are significantly different in play style, the features of their virtual populations can be modeled similarly, allowing future researchers to accurately simulate these types of games.