The investment model is one of the most influential theoretical frameworks for understanding commitment in close relationships. Nonetheless, few studies have examined commitment dynamics using within-persons designs. In addition, there have been few attempts to examine potential antecedents of investment dynamics. The current research attempts to integrate the investment model with contemporary perspectives on attachment and perceived partner responsiveness by examining relationship dynamics within and between persons in a yearlong, intensive longitudinal design. We found that across levels of analysis, perceived partner responsiveness shaped investment model variables which, in turn, shaped commitment. We also found that individual differences in attachment moderated some of these dynamics, such that people who were insecurely attached were less likely than others to perceive their partner as responsive. We suggest taking a more integrationist approach to close relationship research and explore romantic relationship dynamics on the within-person level.