The stability of interindividual differences (i.e., rank-order continuity) in personality traits tends to increase with age until it plateaus in middle adulthood and finally decreases in old age. Rank-order continuity also tends to decrease as the time intervals between occasions of personality assessment increase, irrespective of age. These patterns show that personality development is a lifelong process. Yet the sources of these patterns are unknown. Theories suggest that personality continuity and change may result from environmentally mediated processes of identity development due to age-graded social roles and individual life experiences, but also from biological maturation. Genetically informative longitudinal studies across different age cohorts allow a differentiated picture of genetic and environmental sources. In this article, I give a short overview of the genetic and environmental contributions to rank-order continuity and change in neuroticism and extraversion. Both genetic and environmental factors contribute to personality continuity and change, but genetic factors affect rank-order change only in younger decades of life, whereas environmental influences appear to represent a lifelong source of interindividual differences in personality development.