The intelligence-as-process, personality, interests, and intelligence-as-knowledge (PPIK) framework expands on the notions introduced by Cattell in his investment hypothesis. This framework enables to understand adult intellect in general, and to consider how the interactions among process-type intellectual abilities; non-ability traits, such as affective traits and conative traits; and an individual’s investment of time and effort lead to the development and maintenance of domain knowledge in adulthood. The development of intelligence-as-knowledge in late adolescence and adulthood appears to be related to an individual’s standing on a relatively small number of constellations of non-ability traits, called trait complexes. The introduction of an omnibus test of intelligence specifically designed for adults removes the problems caused by the concept of mental age and by reliance on tests that had been extended from assessment of children. In particular, for both young adults and middle-aged adults, scores on the science/math and intellectual/cultural trait complexes were positively associated with domain knowledge in academic domains, and the social trait complex was negatively correlated with knowledge across nearly all domains. The PPIK theory provides just one perspective on adult intelligence and adult intellectual development among many competing theories.