This text provides a comprehensive treatment of one of the major constructs of behavioural science - general mental ability - labelled the "g factor" by its discoverer, Charles Spearman. The "g factor" is about individual differences in mental abilities. In factor analyses of any and every large and diverse collection of measures of mental abilities, however varied the content of knowledge and skills they call upon, "g" emerges as the largest and most general source of differences between individuals and between certain subpopulations. The author explains the psychometric, statistical, genetic and physiological basis of "g", as well as the major theoretical challenges to the concept. For decades a key construct in researchers in the brain sciences as well as education, sociology, anthropology, evolutionary psychology, economics, and public policy is clearly evident in this treatment of "g".