The topic of human intelligence exceeds the span of any one discipline or method of inquiry. Different aspects of intelligence are best understood from disciplines as diverse as evolutionary biology, neuropsychology, cognitive psychology, anthropology, and education. At its core, however, intelligence is defined by differences between individuals or species. To say that one individual is more intelligent than another is to make a value judgment. Theories of human intelligence must therefore be able to explain those behaviors or accomplishments that societies value as indicants of intelligence (Sternberg, 1985). Such explanations may, at one extreme, invoke the action of neural mechanisms (Garlick, 2002) or, at the other extreme, the importance of social processes (Vygotsky, 1978). Ultimately, however, the theory must explain individual differences in those complex human behaviors that are most commonly understood as indicants of intelligence. Thus, the central facts to be explained by a theory of intelligence must go beyond faster or more efficient processing of elementary tasks, for example, or the efficiency of biological processes and inherited structures, or the influence of schools, environments, or even cultures. Rather, a theory of intelligence must explain the writing of novels, the solving of complex mathematical problems, the designing of skyscrapers and microchips, and the myriad other forms of complex cognition valued by society. In short, an understanding of how individuals solve complex tasks and an explanation of why they differ so markedly in their ability to do so are central facts for any theory of intelligence.