Immigrant youth differ in their adaptation, which is judged on the basis of how well they deal with developmental and acculturative tasks. While immigrant adolescents are faced with the realities of 2 different cultures, they also have to master age-salient tasks, such as self-efficacy and identity development. To get a better insight into the interplay of developmental and acculturative tasks and their relationship with family functioning, we used 3-wave longitudinal data over a 2-year period from 13-year-old immigrant students (N = 609) in Athens, Greece. Cross-lagged models revealed that family functioning and acculturation were resources for the mastery of developmental tasks. Involvement in the host culture prospectively predicted self-efficacy beliefs, and involvement in the ethnic culture prospectively predicted ethnic identity. These effects increased over time. Family functioning prospectively predicted self-efficacy and ethnic identity. These effects decreased over time. The findings suggest that a well-functioning family, for early adolescents, and being involved in the host culture and in ethnic cultures, for middle adolescents, are particularly important resources to master the tasks of their developmental period. Our findings underscore the importance of developmentally sensitive approaches and the need to account for acculturative challenges in order to understand individual differences in immigrant youth adaptation.