Insurgency has been the most prevalent form of armed conflict since at least 1949, as well as the subject of countless historical and contemporary studies. Contemporary discourse on the subject is voluminous and often contentious, but to date there has been a dearth of systematic evidence supporting the counterinsurgency (COIN) approaches, practices, and tenets that make for successful operations. Relying on a collection of the 30 most recent resolved insurgencies, along with a bank of factors that helped or hindered the COIN force in each case and in each phase of each case, several commonalities emerge. For instance, the data show that good COIN practices tend to "run in packs" and that the balance of selected good and bad practices perfectly predicts the outcome of a conflict. The importance of popular support is confirmed, but the ability to interdict tangible support (such as new personnel, materiel, and financing) is the single best predictor of COIN force success. Twenty distinct approaches to COIN are rigorously tested against the historical record, providing valuable lessons for U.S. engagement in and support for COIN operations.