This chapter outlines the main tenets of differential psychology, a research field that addresses individual differences in behaviour and its relevance for central realms such as health, education, and work. The chapter revolves around three main themes. First, it describes the characterization and appraisal of individual differences. Second, it emphasizes the considerable individual variability in the factors and processes involved in playing and mastering chess, with a focus on the intelligence-as-process, personality, interests, and intelligence-as-knowledge theory (PPIK) as an optimal starting point to conceptualize and examine individual differences. Third, the chapter addresses the compelling debate about the heredity versus environment dichotomy to explaining complex human intellectual behaviour, while highlighting its theoretical significance for the development of chess expertise.