Personality development is not only about changes in traits but also about changes in other layers of the self, such as the identity layer. Forming one's identity is thought to be the key developmental task of adolescence, but profound changes in personality traits also occur in this period. In this article, I summarize research on these processes, including research on adolescent personality trait development from a variable-centered and a typological point of view. With regard to identity formation, I also describe contemporary models to measure identity formation and address the importance of examining identity formation on a day-to-day level. Despite obvious theoretical ties between personality traits and identity formation, few studies have examined how these processes affect one another across time.