The exceptional sociability of human life colors nearly every phenomenon in the social and behavioral sciences. However, most psychological theories continue to adopt a within-person perspective, analyzing human behavior by reference to individual-level biological processes, personal dispositions, or cognitive experiences. Interdependence theory is an important antidote to this actor-focused bias. Interdependence theory identifies the most important characteristics of interpersonal situations via a comprehensive analysis of situation structure and describes the implications of structure for understanding intrapersonal and interpersonal processes. Situation structure matters because it is the interpersonal reality within which motives are activated, toward which cognition is oriented and around which interaction unfolds. This paper describes key principles of interdependence structure and processes, and illustrates the utility of an interdependence theoretic analysis via a review of four phenomena – regulatory fit, persistence in the face of dissatisfaction, tit-for-tat versus generosity, and the origins and consequences of trust.