Behavioural genetic research has led to important advances in the field of personality psychology. When carried out on longitudinal data, behavioural genetic studies also offer promising ways to examine the genetic and environmental origins of personality stability and change. Here, we review the findings of longitudinal twin studies, discuss their implications for our understanding of adult personality development, and point out open questions that need to be addressed by future research. Three general conclusions stand out. First, there is a strong and relatively stable genetic foundation of individual differences in personality throughout the adult life span; second, environmental influences become more important and contribute to an increasing rank–order stability of personality traits from early to middle adulthood; and third, both genetic and nonshared environmental influences contribute to both stability and change in personality traits. Equipped with this knowledge, the most urgent tasks for the next generation of behavioural genetic studies on personality development will be to (i) identify measurable environmental factors that matter and (ii) to capture the interplay between genetic and environmental influences on personality stability and change throughout adulthood. Copyright © 2014 European Association of Personality Psychology