Open enrollment in public schools is a form of school choice that allows students to attend schools other than the one assigned to them by their school district. Though often less visible than policies such as charter schools, vouchers, and education savings accounts, K-12 open enrollment is rising in popularity across the nation, and 73 percent of school parents support it. As of 2023, 43 states permit or mandate some degree of open enrollment, but only 16 states have strong open-enrollment laws. Since 2021, 10 states have significantly improved their open-enrollment laws. For example, Idaho's new law requires all school districts to participate in open enrollment and also establishes better program transparency. When it comes to open-enrollment data, however, researchers and policymakers are often left in the dark. Only 13 states are required by law to collect data on open enrollment, and only three states publish these figures regularly. As a result, little is known about a key policy that affects students and public schools nationwide. There are two types of open enrollment: cross-district open enrollment allows students to attend schools outside their school district, while within-district open enrollment lets students attend schools outside their assigned zone but within their own school district. To understand the role these programs play in the school choice landscape, the author obtained data from three states--Arizona, Florida, and Wisconsin--that host some of the most robust open-enrollment programs in the nation. Participation is strong; more than 450,000 students in these three states used open enrollment to attend public schools other than their assigned ones during the 2021-22 school year.