The development of adult intelligence assessment early in this century as an upward extension of the Binet-Simon approach to child intelligence assessment is briefly reviewed. Problems with the use of IQ measures for adults are described, along with a discussion of related conceptualizations of adult intellectual performance. Prior intelligence theories that considered adult intelligence (Cattell, 1943, 1971/1987; Hebb, 1941, 1942, 1949; Vernon, 1950) are reviewed. Based on extensions of prior theory and new analyses of personality-ability and interest-ability relations, a developmental theory of adult intelligence is proposed, called PPIK. The PPIK theory of adult intellectual development integrates intelligence-as-process, personality, interests, and intelligence-as-knowledge. Data from the study of knowledge structures are examined in the context of the theory, and in relation to measures of content abilities (spatial and verbal abilities). New directions for the future of research on adult intellect are discussed in light of an approach that integrates personality, interests, process, and knowledge.