The structure of this report is as follows: First, specific aptitude theory and general ability theory, the two theories of the relation between ability and job performance, are presented and differentiated. Second, there is a discussion of problems in the current use of the General Aptitude Test Battery (GATB) and of problems in the use of multiple cutoff scores based on small sample data. Third, there is a discussion of the dimensionality of the GATB in traditional factor analytic terms. That is, there is an analysis of the correlations between specific aptitude scores over persons. This culminates in a breakdown of each of the nine specific aptitudes and the three general ability composite scores in terms of general factor variance, specific factor variance, and error variance. Fourth, there are the data on the correlation of aptitude validity coefficients across jobs. These data tend to support the general ability theory. Further evidence for the general ability theory is presented in connection with spatial aptitude. The practical implication of these findings is that the search for applications of specific aptitudes to the prediction of job performance will require either very large sample size studies (N=1,000) for particular jobs or the identification of special job families. (PN)