In his long career, Donald T. Campbell made important contributions to social psychology, anthropology, sociology, education, science studies, and epistemology. In this anthology, the authors concentrate on his epistemology, in particular his evolutionary, naturalistic epistemology. The four philosophers, two psychologists, a sociologist, and specialists in science studies and education discuss Campbell's contributions, explaining and criticizing them in a comprehensive way. Campbell and his ideas are treated in a strikingly new light—Campbell enters the new millennium.