Adolescents need to develop their own identities in order to become responsible adults and citizenships. Furthermore, formation of a firm identity is necessary to promote adolescent mental health and to prevent endorsement of risk and delinquent behaviours.The purpose of this project is to examine identity formation over time and to investigate antecedents and consequences of adolescent identity changes. In order to overcome current shortcomings of the identity research field this project will (a) use as framework a recent process model of identity aimed at disentangling the dynamic by which identity is formed and revised over time; (b) analyze identity formation in junior and high school students; (c) adopt longitudinal designs to directly test causality links involving identity changes, their predictors, and their effects; (d) take into account contextual influences that may impact adolescent identity; (e) examine ethnic differences, to test whether immigrant adolescents living in the Netherlands face the identity formation task differently from their autochthonous peers; (f) investigate gender differences, to find peculiarities and similarities between boys’ and girls’ identity paths; (g) translate main findings in meaningful recommendations for practitioners and policy makers that could use evidence-based results to plan interventions aimed at promoting healthy adolescent development.This project includes three studies aimed at addressing the following research questions: how does identity develop over time? Which are the effects of identity transitions on depressive and anxiety symptoms? (Study I); Which are the life events that can be predictive of identity changes over time? How does early adolescent identity predict middle and late adolescent developmental paths? (Study II); Do dyadic adolescent-best friend relationships influence identity formation and how (i.e., through identification or differentiation dynamics)? (Study III).