Recently, MMOs and social games have begun pushing the envelope by adding rich AI agents to fill out their human-driven worlds. This movement exposed a new set of challenges. How do you create and manage thousands of simultaneous MMO agents that may interact with tens of thousands of users? And how do you do it in a manner that allows the fast-paced and often iterative release schedule of today's online games? This lecture will outline the challenges unique to architecting AI for online games, and explain some of the solutions used by Zynga's CityVille and ArenaNet's upcoming Guild Wars 2.