Differential psychologists rightly identified evolutionary theory as a unifying framework for explaining the origins and persistence of individual differences in a wide array of human psychological characteristics. Psychological diversity occurs on multiple levels, including between species, populations, generations, and individuals. Each level reveals the outcome of evolutionary processes at different temporal scales. I embrace a range of methods and results from quantitative and population genetics, developmental evolution, and phylogenetically grounded comparative psychology to explore how personality evolves in humans and nonhuman primates. At the level of species, I compared personality structure derived from rater assessments for four species of macaques and found a consistent, core set of personality dimensions (Dominance, Confidence, and Friendliness) describing these species. At the population level, I studied the relationship in humans between fertility/longevity trade-offs and the average personality of a country and found that Neuroticism and Agreeableness exhibit adaptively plasticity to life-history conditions. At the level of families, I estimated the quantitative genetic structure of personality in orang-utans and found that, like humans, a large portion of the phenotypic variance was explained by non-additive genetic effects. I examined between generation changes in personality by testing whether personality traits in humans are genetically correlated with fitness and found that in modern environments personality evolves very slowly. Finally, I translated current conceptual models of biological reactivity and stress response into mathematical models of developmental evolution and determined that evolution would select highly resilient phenotypes but that variation could be maintained by skew in the distribution of underlying genetic factors. From these results I broadly conclude that primate personality structure is generally conserved among species, mean personality levels change only very slowly between human generations, and that this evolution results in a genetic basis of personality that is characterized by epistasis. The evolution of individual differences has much to gain from the rigorous application of evolutionary methodology.