The relationship between general cognitive ability and both training and job performance is reviewed. Existing scientific data show that there are large differences in training achievement and in job performance. Consequently, any good predictor of achievement or performance can yield a large gain in workforce productivity. General cognitive ability (a phrase that is preferred to "intelligence") is the best known predictor of job performance and training success. A review of existing studies indicates that the Wonderlic Personnel Test (and its equivalent form the Wonderlic Scholastic Level Exam) is job-related, and has a higher validity with regard to predicting training success and job performance than any alternative predictor. It is fair to minority applicants, and does not understate their abilities. Evidence that selection using the Wonderlic can yield a large increase in workforce productivity is reviewed. Use of the Wonderlic Exam is justified on scientific and practical grounds. There are 12 tables of supporting data and 1 illustrative figure. A 53-item list of references is included. (SLD)