Understanding Non-State Actors aims to reduce the scarcity of academic literature on armed non-state actors (NSAs) that have always been a part of world politics and wars. This monograph offers, possibly for the first time, a systematic historical review as well as a substantive theory of NSAs and their arming efforts. From the Jewish rebellions against Rome to the war between the Ukrainian separatists and the Ukrainian government, NSAs’ weapons acquisition has been vital for the build-up of their force, enabling both the employment of that force and its sustainability. While weapons are not necessarily the most important factor in military build-up, NSAs need weapons to fight, and revolts usually erupt after the organizers have acquired a certain number of weapons. Conversely, many revolts lose momentum and operations are not carried out, or turn ineffective, due to shortages of arms and ammunition. A major theme of this monograph is that in spite of dramatic political and technological changes, armed NSAs in different periods have employed similar methods to acquire weapons. Self-production, looting and stealing, external support, and the arms trade were always the major ways for NSAs to acquire weapons, though the importance of each method and the type of arms has changed remarkably over time. Understanding Non-State Actors discusses the factors – political, social, cultural, technological, and organizational – that have both facilitated and constrained the ability of NSAs to acquire arms. Especially, lecturers and students of Military, Terrorism, Conflict studies, War and peace studies will benefit from this study.