Do spouses become more similar over time? What processes contribute to enduring similarities between them? Using the 20-year Kelly Longitudinal Study of couples, no support for the hypothesis that couples increasingly resemble each other with time was found. Rather, couples maintain the same degree of similarity across 20 years. Structural equation analyses suggest that the shared environmental experiences of couples play a significant role in maintaining these similarities over time. We distinguish the shared marital environment from the shared rearing environment and consider developmental and dynamic-relational factors that moderate the relative importance of nonshared and shared environmental experiences in life-span personality development. Whereas nonshared influences in one's family of origin contribute to development in childhood and adolescence, shared influences in one's family of destination may contribute a great deal to development in adulthood.