This study examines the effectiveness of traditional admissions criteria, including prior GPA, SAT, GRE, and TOEFL in predicting undergraduate and graduate academic success for American and international students at a large public university in the southwestern United States. Included are the admissions and enrollment data for 25,017 undergraduate American, 509 undergraduate international, 5,421 graduate American, and 1,733 graduate international students enrolled between 2005 to 2009. Person product-moment correlation, multiple regression, and user-determined stepwise regression were applied to the data. Results show high school GPA is the most predictive of first-year college GPA for both undergraduate American and international students. SAT has a medium correlation with first-year college GPA for American students and a large correlation for international students. High school GPA and SAT together explain one fourth of the variance in first-year college GPA for American students and over one half of the variance for international students. TOEFL has a medium correlation with first-year GPA for undergraduate international students but is not a significant predictor of first-year GPA when SAT is included in multiple regression. Unlike the results for undergraduate students, the traditional admissions criteria (undergraduate GPA and GRE) for graduate admissions explain a small portion of variance in first-year graduate GPA. Undergraduate GPA, GRE Verbal, and Quantitative together explain 6.3% of variance in first-year graduate GPA for American students and 3.1% for international students. The GRE Subject Tests are the best predictor of first-year graduate GPA for students who had taken the GRE Subject Tests. TOEFL has a small correlation with first-year graduate GPA for international students, and it is not a significant predictor of graduate GPA when GRE-Verbal is included. These findings have implications for undergraduate and graduate admissions, standardized admissions tests, university curriculum, and students' academic success. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]