The social psychologist and generalist Donald T. Campbell has contributed to psychology and the social sciences in many important ways. This entry first describes his approach to evolutionary epistemology, including his General Selection Theory, then his work in the philosophy and sociology of science, and finally his work in research methodology (with an emphasis on his development of quasi-experiment design and conceptual validity). He described his philosophical position as naturalist, ontological critical realist, epistemological relativist, constructivist, fallibilist, and anti-foundationalist and as adhering to a correspondence theory of truth, although he saw coherence theory as a necessary substitute. Apart from his work on quasi-experimental design, he is best known for his blind variation, selection and selective retention (BVSSR) model approach to knowledge change, which he used to account for human knowledge abilities, including humans’ ability to use science and culture in general to influence their evolutionary fit.