The role of behavior observation in theory-driven prevention intervention trials is examined. A model is presented to guide choice of strategies for the measurement of five core elements in theoretically informed, randomized prevention trials: (1) training intervention agents, (2) delivery of key intervention conditions by intervention agents, (3) responses of clients to intervention conditions, (4) short-term risk reduction in targeted client behaviors, and (5) long-term change in client adjustment. It is argued that the social processes typically thought to mediate interventionist training (Element 1) and the efficacy of psychosocial interventions (Elements 2 and 3) may be powerfully captured by behavior observation. It is also argued that behavior observation has advantages in the measurement of short-term change (Element 4) engendered by intervention, including sensitivity to behavior change and blinding to intervention status.