A theory of the nature of mental abilities is presented. In this theory, mental abilities are hierarchically organized into four progressively deeper levels--the levels of composite tasks, subtasks, information-processing components, and information-processing metacomponents. Composite tasks can be decomposed into subtasks, subtasks into components. Metacomponents control the use of components in composite tasks and subtasks. Each of the four levels of mental abilities is described and interrelated to the others. The fundamental theoretical questions relevant at each level are posed, and answers to these questions are proposed. The role of factors in the theory is described, and is shown to be quite different from the role of factors in traditional theories of mental abilities. Full understanding of mental abilities requires understanding of all four levels. (Author)