Interdistrict open enrollment is the most widely used form of school choice in the United States. Through the theoretical lens of a utility maximization framework, this article analyzes the determinants of interdistrict open enrollment flows in Minnesota and Colorado. The authors' empirical analysis employs an original data set that details open enrollment flows between all pairwise combinations of school districts within 100 miles of each other in these two states. These flows are merged with demographic and geographic data from the Common Core of Data and U.S. Census Bureau. The findings indicate that open enrollment flows are driven mainly by student achievement and structural characteristics of districts; distance plays a large constraining role. The results also suggest that most transfers take place between relatively high-achieving districts. The authors discuss the policy implications of these findings. (Contains 23 notes, 6 tables, and 2 figures.)