Momentum is growing around a proposed treaty governing the international transfer of small arms and light weapons. Those promoting the new instrument emphasize the existing obligations of States, under the law of State responsibility, not to aid or assist another State in violating international law. This article explores the extent to which the prohibition of “complicity” is a sufficient basis for requiring States to consider the end-use of the weapons they transfer. It offers suggestions for strengthening the effectiveness of the current draft treaty in a way that places respect for international humanitarian law and human rights at its core.