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. Ideologically driven violent extremists are a primary threat to national and human security in the developed and developing worlds, suggesting that, in some form or other, CVE will remain on the counterterrorism agenda in the short and medium terms. 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 advances this objective by asking, “Does CVE work?”