This paper presents a brief history of the work of Professor Karl J. Holzinger in developing, over a 32-year period, the basic concepts of factor theory. The discussion includes early factor studies, including Spearman's two-factor theory; development of the bi-factor method; characteristics and uses of bi-factors; the development of the Holzinger-Crowder Uni-Factor Tests (a battery of nine subtests that measure four factors—verbal, spatial, numerical and reasoning); and Holzinger's concern for the factorial layman—the simplicity of his language and consequent accessibility of his writings to non-mathematicians.