The first, and most important, is the necessity of distinguishing between intelligence in the conceptual sense, which the author takes to mean individual differences in cognitive competence, and intelligence in the much narrower sense of the trait that is measured by conventional cognitive tests. This chapter is divided into four sections. The first three sections of the chapter discuss developments in the measurement of intelligence, the causes of intelligence, and the implications of having or not having intelligence. The final section deals with demographic issues. Throughout the chapter the author focuses on intelligence within the normal and superior range. Changes in intelligence with age, differences between men and women, and differences across racial/ethnic groups all raise scientific and social policy issues. One of the most important issues is to provide a physiological explanation for individual differences in neural processing speed.