In the past few years, dozens of states, localities, and schools have proposed or enacted "free college" policies, also known as College Promise programs. While a handful of states have run Promise-like programs for decades, the 2014 launch of the Tennessee Promise and the Obama administration's focus on the concept catalyzed several states to pursue their own versions of a statewide free or debt-free college proposal. This report first identifies and analyzes the range of design choices made by states and the challenges and opportunities created by those choices, finding that states that have enacted Promise programs since 2014 have imposed significant eligibility requirements. It then provides guiding principles to help states considering future Promise programs to avoid inequitable cost containment mechanisms, maintain a clear message to students and families, and ultimately make bolder investments in higher education.