In this article, we present an integrative perspective on temperament and personality development. Personality and temperament are conceptualized as regulatory systems that start as physiological reactivity to environmental features early in life, but are increasingly supplemented by regulation efforts oriented toward reference values such as personal goals and social norms. These reference values change during development as society expects increasingly mature behaviors, but it takes regulatory resources and incremental practice before people can conform to these higher standards. Consistent with this view, a meta-analysis of mean-level development of personality traits in adolescence revealed a decrease in conscientiousness and openness during early adolescence. Negative discrepancies between reference values and actual behavior are apparently responsible for decreases in perceived maturity, but more direct evidence is needed to support this claim.