There is a substantial body of research showing that teacher quality is an important determinant of student achievement in elementary and secondary schools, inspiring some states and districts to enact policies aimed at identifying and rewarding high-quality teachers. Yet relatively little is known about the impact of instructor effectiveness on student performance in higher education, where such insights could be particularly useful. Even more than leaders at K-12 schools, college administrators often have substantial discretion to determine which instructors receive teaching assignments. This lack of research is largely the result of data and methodological challenges. Whereas K-12 schools administer standardized tests to most students in core academic subjects, there are few such common assessments at colleges--even among students taking the same course at the same campus. In addition, college students are able to choose their classes and thus, their instructors. Because college students have a great deal of flexibility compared to students in K-12, simply comparing their success rates across instructors is likely to be misleading. In this study, the authors overcome these challenges by examining data on more than 2,000 algebra instructors at the University of Phoenix (UPX), a for-profit institution that is the largest university in the United States. UPX follows a unique instructional model based on common, standardized curricula and assessments, in both online and face-to-face classes. These assessments provide an objective outcome by which to measure instructor effectiveness, and the authors use them to examine two questions: (1) How much does student performance vary across instructors?; and (2) is instructors' effectiveness correlated with their teaching experience and salary? The authors find substantial variation in student performance across instructors, both in the instructor's class and in a subsequent class. Differences are substantial in both online and in-person courses, though they are larger for in-person classes. Notably, instructor effects on students' future course performance are not significantly correlated with student end-of-course evaluations, the primary metric through which instructor effectiveness is currently judged. Findings suggest that colleges could improve student outcomes by paying more attention to who is teaching their classes.