Human intelligence has an intimate relationship with the anatomical structures and physiological functions of the nervous system, and psychologists often refer to the brain as the basis for or substrate of intelligence. At the same time, intelligence is a very complex psychological phenomenon that is not easily understood in terms of the properties of nerve cells and brain circuitry. Several significant neural correlates of IQ test scores have been documented, but it is not at all clear that variation in IQ is actually caused by variation in these aspects of the nervous system. Genetic variation is undoubtedly important for individual differences in both brain structure and intelligence, yet mounting evidence also demonstrates that differences in environment have major influences on both brain structure and intelligence. A limited set of genes or a distinct array of features of the nervous system cannot encapsulate the concept of intelligence or provide a better way to measure intelligence. Whether population variance in intelligence arises largely from a single, general factor (g) or from many specific factors therefore is not a question that can be answered decisively by examining the brain or the genes. The present chapter does not purport to refute or affirm the reality of g as a psychological concept. Rather, it argues against the theory of biological intelligence-the assertion that intelligence is essentially a genetically determined biological entity.