Despite the relative stability of personality, multiple studies were able to show that personality develops across the whole life span with unique developmental patterns occurring in the time of emerging adulthood. Rank-order consistency has been shown to increase substantially, mean-level changes in the direction of the maturity principle (i.e., increases in emotional stability, agreeableness, and conscientiousness) have been repeatedly demonstrated, and interindividual differences in change were shown to be most pronounced. Aiming to explain these findings, social relationships and work haven (...) been shown to be important environmental contexts for personality development. However, some questions have remained open. First, little has been understood about the personality development of emerging adults undergoing a different educational pathway than college education. Therefore, study 1 investigated personality development in emerging adults undergoing 3-year vocational education and training. The study showed the young apprentices to develop counter the maturity principle. Second, innovative theoretical and methodological approaches to more adequately model dynamic transactionism theory in personality-social relationship transactions have been suggested to overcome the imbalance of effects. Thus, study 2 investigated personality-relationship transactions in a major life transition from high school to post-secondary education and beyond. However, the dominance of personality effects on subsequent social relationship development was reinforced for emerging adulthood. Third, following the ongoing debate on possibilities to psychologically assess environmental contexts, study 3 tested multiple hypotheses on the interplay between the importance ascribed to basic psychological need support (BPN), perceived BPN support, and personality development. The findings revealed the individual’s importance ascribed to BPN to primarily important for personality development. (shrink)