James Mark Baldwin is one of the most important and least known early American scientific psychologists. Drawing inspiration from Charles Darwin and other evolutionists of the period, Baldwin developed a biosocial theory of psychological development that influenced both Jean Piaget and Lev S. Vygotsky; and he proposed a mechanism relating learned adaptations in the individual to phylogenesis (frequently termed the "Baldwin effect") that is of considerable interest to those currently modeling processes of learning and evolution. After a brief introduction to Baldwin's career, this article describes the intellectual context within which his evolutionary thinking developed. Three of his most important contributions are then discussed: his theory of individual adaptation or learning, his concept of "social heredity," and his articulation of the "Baldwin effect." The article concludes with a brief evaluation of the contemporary importance of Baldwin's ideas. (Contains 5 footnotes.)