This evidence review paper examines the most effective strategies for international actors to provide support for nonviolent action combines, based on a comprehensive examination of over 2,000 published studies related to nonviolent action from leading journals, organizational reports, and university press books. Its central finding is that, while the field is growing, many research gaps remain and much international assistance is conducted based on theories of change that have weak or nonexistent evidence bases. The paper calls for greater consideration of the complex ethical considerations at the heart of external support for nonviolent action and for careful, rigorous research into the intended and unintended effects of such support.