In research conducted "over a period of 20 years" on aptitudes of high level personnel, the purpose was to analyze thinking abilities or processes into their unique components. The approach was through study of individual differences in performance on intellectual tasks varied systematically over broad ranges of psychological functioning. Thirty-six major factor analyses were carried out; the main goal was to achieve invariance of psychological factors to represent unique intellectual abilities. An unusually high level of invariance was accomplished. The most significant outcome was the development of the structure-of-intellect theory and model. A three-dimensional matrix, this model has logical places for 120 unique abilities or functions, of which 98 had been demonstrated by the end of the program, 22 not having been investigated. The model has numerous implications beyond its immediate value of pointing toward functions yet to be investigated. It provides some significant "new looks" at general psychological theory, some guides for development of intelligency by educational practices, and some new vistas in aptitude assessment. (Author/NL)