This chapter focuses on individual differences in personality, because differences among individuals are the most remarkable feature of human nature. After all, in both genetic and cultural evolution, selection pressures operate on differences among people. Not surprisingly, individual differences pervade all aspects of life, and they demand scientific inquiry: What are the most salient personality differences between people? What gives rise to these differences? Do personality differences shape important life outcomes? How might personality influence the emergence of psychopathology? Answers to these questions are crucial for those who wish to describe, explain, and predict the nature of individual lives across time.