Personality traits continue to change throughout the lifespan. However, we still know little about when, why, and how personality traits change. In this paper, we review the current state of scientific evidence regarding the nature, sources, and processes of personality trait stability and change. We revisit past disputes over the relative importance of genetic and environmental influences, discuss studies on life events and personality trait development, and summarize theory and research on personality change processes. In doing so, we derive general principles of personality trait development, highlight limitations of past research, and present the broad outlines for future research on personality trait development, with a particular emphasis on relevant methodological issues and conceptual challenges.