The question of whether the person or the situation is largely responsible for behavior has plagued psychology intermittently for the last half century. Studies of the heritability, stability, and consensual validity of traits have clearly demonstrated the existence of traits. However, there is continuing controversy about the role of traits and situations in the enterprise of personality psychology. The goal of this article is to describe how insights yielded from adopting a developmental approach can foster the successful integration of the person and the situation across the life span. Five key lessons are described: (a) age matters--studying different age groups can lead to biases for and against traits and situations; (b), if age matters, time matters more--longitudinal and within-participant designs demonstrate that traits and situations are reciprocally related; (c) examine multiple types of change--focusing on one type, such as mean-level change, can lead to inappropriate conclusions about the merits of persons or situations; (d) be sensitive to levels of analysis--the relative breadth of persons and situations may determine the relative influence of the two; (e) pay attention to process--process models lead inextricably to transactional explanations.