This chapter argues that the modeling personality in terms of the behavior of organized motivational systems can help unify the structural and dynamic approaches. It demonstrates how structured motivational systems can transform variability in chronic motivations across individuals into patterns of behavior from which the Big Five can be recovered. The chapter discusses how both within- and between-person variability in personality-related behavior can be understood in terms of the dynamics of the interaction between individual's motives, the affordances of situations, and current bodily states. In a series of recent experience-sampling studies of everyday behavior, Fleeson has shown that while there are clear individual differences, consistent with the Big Five, in trends in personality related behavior over time, at the same time the within person variability over situations is at least as large as the between person differences. The chapter focuses on the role of motive affordances in explaining why people may behave quite differently in different situations.