This chapter provides an overview of illicit trafficking of small arms and light weapons and their ammunition, and relevant research. We first describe two implicit debates within scholarship on arms availability. Then, we explain the different forms of the arms trade (including wholly illicit and government-complicit transfers), the likely extent, and the supply and demand of illicitly trafficked arms. Since arms are a durable and tradeable good, when weapons are diverted to the illicit market it creates a pool of illicit arms that can recirculate until removed by government measures. Controlling the means of violence – including arms – is one of a state’s basic functions, and an important component of fragility is whether a state is able and willing to prevent arms trafficking into and within its borders. The patchwork of laws, regulations, and enforcement globally provide loopholes and pathways that allow for trafficking. With the Arms Trade Treaty and other agreements, the international and various regional communities have attempted to set forth regulations and implement capacity-building measures in order to counter arms trafficking. Such anti-trafficking measures include assistance for physical, legal, and social efforts.