At the intersection of new weapon technologies and international humanitarian law, so-called “non-lethal” weapons have become an area of particular interest. This article analyses the relationship between “non-lethal” weapons and international law in the early 21st century by focusing on the most seminal incident to date in the short history of the “non-lethal” weapons debate, the use of an incapacitating chemical to end a terrorist attack on a Moscow theatre in October 2002. This tragic incident has shown that rapid technological change will continue to stress international law on the development and use of weaponry but in ways more politically charged, legally complicated and ethically challenging than the application of international humanitarian law in the past.