As a field of policy and practice, countering violent extremism (CVE) has emerged rapidly in recent years and represents the most significant development in counterterrorism over that time. But CVE stands at something of a fulcrum point. There is enough experience in "doing CVE" to expect that data about its effects and effectiveness can be gathered and analyzed. In turn, such analyses ought to inform future developments if policy is to be evidence based. This report asks, "Does CVE work?" In elaborating a response, the report provides a brief primer on CVE, which is often criticized for lacking coherence as a field. It reviews publicly available evaluation research on CVE to derive lessons from the past, which pertain to governmental efforts to engage communities for the purpose of CVE. It analyzes how those evaluations and related efforts to learn from experience have impacted the evolution of CVE, which has, in general, become better focused on the risk of extremist behavior over time. In turn, the report reviews the state of the art in evaluating CVE measures. The report makes the case for systematizing the understanding and practice of CVE, committing to its evaluation, and moderating expectations about its impacts.