While the overall stability of personality across the lifespan has been well-documented, one does see incremental changes in a number of personality traits, changes that may impact overall life trajectories in both positive and negative ways. In this chapter, we present a new, developmentally-oriented and integrative model of the factors that might lead to personality change, drawing from the theoretical and empirical work of prior models (e.g. Caspi & Roberts, 2001; Roberts et al., 2005) as well as from our own longitudinal studies of personality change and risky behavior engagement in children, adolescents, and young adults (Boyle et al., 2016; Riley & Smith, 2016; Riley et al., 2016). We focus on change in the trait of urgency, which is a high-risk personality trait that represents the tendency to act rashly when highly emotional. We explore processes of both biologically-based personality change in adolescence, integrating neurocognitive and puberty-based models, as well as behavior-based personality change, in which behaviors and the personality traits underlying those behaviors are incrementally reinforced and shaped over time. One implication of our model for clinical psychology is the apparent presence of a positive feedback loop of risk, in which maladaptive behaviors increase high-risk personality traits, which in turn further increase the likelihood of maladaptive behaviors, a process that continues far beyond the initial experiences of maladaptive behavior engagement. Finally, we examine important future directions for continuing work on personality change, including trauma-based personality change and more directive (e.g., therapeutic) approaches aimed at shaping personality.