This article examines the obligations contained in the Arms Trade Treaty for preventing diversion, which is defined as the movement from authorized to unauthorized possession or use. It argues that in different places, the treaty covers both diversions taking place during a transfer and after the transfer has been completed. The article then considers the different ways that authorization can be granted; it points out that a diversion can occur if any state involved in a transfer has not provided authorization. The article then presents three examples of diversion in contemporary armed conflicts: the 2011 war in Libya; supplies to Afghanistan and Iraq between 2003 and 2016; and an export of small arms and ammunition from China to Nigeria, which was brokered from the United Kingdom. In each example, the nature of a diversion is highlighted, as are the specific issues for implementation of the Arms Trade Treaty. The article concludes with the observations that states parties need to control the activities of arms brokers, much more knowledge is needed by states parties to help them predict when a diversion may occur, and a key priority for international cooperation and assistance should be to enhance stockpile security and management.