Instructors are a chief input into the higher education production process, yet we know very little about their role in promoting student success. This is in contrast to elementary and secondary schooling, for which ample evidence suggests teacher quality is an important determinant of student achievement. Whether colleges could improve student and institutional performance by reallocating instructors or altering personnel policies hinges on the role of instructors in student success. In this paper we measure variation in postsecondary instructor effectiveness and estimate its relationship to overall and course-specific teaching experience. We explore this issue in the context of the University of Phoenix, a large for-profit university that offers both online and in-person courses in a wide array of fields and degree programs. We focus on instructors in the college algebra course that is required for all BA degree program students. We find substantial variation in student performance across instructors both in the current class and subsequent classes. Variation is larger for in-person classes, but is still substantial for online courses. Effectiveness grows modestly with course-specific teaching experience, but is unrelated to pay. Our results suggest that personnel policies for recruiting, developing, motivating, and retaining effective postsecondary instructors may be a key, yet underdeveloped, tool for improving institutional productivity. [This paper was prepared for the NBER Conference "Productivity in Higher Education" held on June 1, 2016.]