Developing a stable and coherent identity structure (i.e., a synthesized sense of self that can support self-directed decision making) represents a lifelong task. A person’s identity structure is continually revised through ongoing processes of identity exploration and commitment. However, longitudinal studies linking identity structure to identity processes are largely lacking. The present three-wave longitudinal study among 530 Flemish high school students [50.6% female; Mage = 15; SD = 1.85; age range = 11–19 years] examined the directionality of both between- and within-person associations linking identity synthesis and confusion to identity exploration and commitment processes. Between-person cross-lagged models indicated that adolescents who scored high on identity synthesis relative to their peers also scored high on proactive exploration and commitment processes, and low on ruminative exploration one year later, again relative to their peers. Adolescents who scored high on identity confusion relative to their peers also scored high on ruminative exploration one year later, again relative to their peers. With respect to effects of identity processes on identity structure, adolescents who scored high on identification with commitment relative to their peers also scored low on identity confusion one year later. Within-person cross-lagged models indicated that, when adolescents scored high on identity synthesis relative to their own average score, they reported increased proactive exploration processes one year later. In general, reaching a degree of identity synthesis appears to represent a prerequisite for proactive identity exploration at both the between- and within-person levels.