Abstraction is one facet of intellectual functioning. The study of abstraction allows extremely valuable insights into human intelligence. While this monograph indicates that the ability to think abstractly declines slightly with age, there are a number of variables determining abstract thinking and its relation to intelligence over the life-span. This monograph defines abstraction from all angles of thought, contrasting it with high-order thinking and stereotyped thinking; it discusses and evaluates tests of abstract thinking; and it presents new findings in sociological and psychological research on abstraction.