This chapter discusses research at the intersection of behavior genetics and the study of the development of personality. These areas are often quite distinct, both conceptually and methodologically. Behavior genetics, in many cases, represents the study of the sources of individual differences at a specific point in time. Moreover, it is often assumed that documenting genetic effects at a specific time implies genetic effects on continuity across time. This is not necessarily the case: Both genetic and environmental forces can influence both change and continuity. Hence, the study of development may be incomplete without disentangling genetic and environmental influences; similarly, the study of genetic and environmental influences may be incomplete without an understanding of how those influences act, interact, and correlate over time.