When we observe others in motion, we usually care little about the surface behaviors they exhibit. What matters are their underlying intentions. Judgments about intentions and intentionality dictate how we understand and remember others' actions, how we respond, and what we predict about their future action. A generative knowledge system underlies our skill at discerning intentions, enabling us to comprehend intentions even when action is novel and unfolds in complex ways over time. Recent work spanning many disciplines illuminates some of the processes involved in intention detection. We review these developments and articulate a set of questions cutting across current theoretical dividing lines.