Correlational evidence suggests that high school GPA is better than admission test scores in predicting first-year college GPA, although test scores have incremental predictive validity. The usefulness of a selection variable in making admission decisions depends in part on its predictive validity, but also on institutions' selectivity and definition of success. Analyses of data from 192 institutions suggest that high school GPA is more useful than admission test scores in situations involving low selectivity in admissions and minimal to average academic performance in college. In contrast, test scores are more useful than high school GPA in situations involving high selectivity and high academic performance. In nearly all contexts, test scores have incremental usefulness beyond high school GPA. Moreover, high school GPA by test score interactions are important in predicting academic success. (Contains 5 figures, 4 tables, and 4 footnotes.)