It needs no great scientific insight to see that such multitudinously multi­ variate subjects as psychology, physiology, sociology, and history need multivariate methods. As this book may show, those methods-multivari­ ate analysis of variance, regression analysis, typal and discriminant func­ tion analysis, multidimensional scaling, and factor analysis-belong to a single structural arch which bears up conceptual and causal understanding in all these subjects. But factor analysis is the keystone of that arch. Since factor analysis has itself developed fantastically in thirty years [my first small book (Cattell, 1952b) could almost cover the field in a few chap­ ters!] , this book confines itself to that subject, with only brief connecting asides on the related areas. The purpose of a preface is to permit more personal comments and to explain why the design is what it is. Among the former the author often apologizes for writing in an already crowded library shelf, and, having con­ fessed the crime, thanks his friends for their connivance. The area already enjoys a truly excellent array of books, from extremely good elementary introductions by Child, Henrysson, Lawlis, and Chatfield, through the workbook emphasis of Fruchter, and the intermediates of Guertin and Bailey and Comrey, to the comprehensive technical works of Anderson, Ahmavaara, Gorsuch, Harman, Horst, Lawley and Maxwell, Mulaik, Rao, Rummel, and Van de Geer, not to mention the undating books by Burt, Thomson, and Thurstone.