Military effectiveness is defined as the ability to produce favorable military outcomes per se, incluuding the outcomes of minor skirmishes at the tactical level of war and the outcomes of wars or even long-term politico-military competitions at the strategic or grand strategic levels of war. An alternative, narrower definition equates “effectiveness” with skill, the ability to make the most of one’s material resources, or qualities such as “integration” and “responsiveness.” Regardless of definition, military effectiveness is a central issue for international relations and lies at the heart of key policy debates. Until very recently, military effectiveness had generated little sustained interest from scholars. However, a new generation of academics, armed with new methodologies and analytical approaches, has begun to pay more attention to the subject. Contemporary literature on effectiveness describes three classes of candidate determinants: numerical preponderance, technology, and force employment. Despite increasing attention to effectiveness per se and to non-material contributors to effectiveness, especially force employment, many topics deserve further consideration in future research. These include maritime warfare, amphibious warfare, space warfare, and cyber warfare, chemical, biological, or nuclear combat operations; effectiveness in non-combat missions such as peacekeeping, nation building, signaling, or humanitarian assistance; systematic differences in military behavior for non-state actors, or for state actors in the developing world; and the roles of organization, logistics, leadership, morale, ethnic homogeneity, civil–military relations, and social structure, for example, as determinants of military effectiveness.