This paper summarizes the research on the relationship between teacher unionization and educational outcomes at the state, district, school, and individual (student) levels. Although teachers are the largest organized professionals in the United States, much of educational policy literature has ignored unionization as a subject of study. An emerging consensus from the literature believes that collective bargaining raises teacher pay, increases district expenditure and reduces class size; however, union influence on student outcomes has not yet been established. The literature is unclear as to whether or not teacher unionization is associated with student graduation rates or standardized test scores.