This paper argues for the scientific utility of the concept “intelligence”. In the first section three common arguments against the validity of general intelligence are discussed and dismissed. The second section presents the evidence in favour of the proposition that individual differences in IQ may be based on differences in speed of information processing. However, the third section shows that executive functions, particularly inhibitory processes, represent a more likely basis for the development of intelligence. The theory of the minimal cognitive architecture underlying intelligence and development (Anderson, 1992a) shows how speed and executive functioning might represent two dimensions to g—one an individual differences (within age) dimension based on speed and the other a developmental dimension based on changing executive functioning. In the fourth section this theory is used to generate new insights on the nature of intellectual disability and specific cognitive deficits and to make practical suggestions for educational intervention for low-IQ children.