This insightful book analyses the issue of norm erosion in international law by examining arms transfers to non-state actors. Balancing empirical research with legal theory, the author dissects recent case studies, tracing individual changes in norms against a background of systemic transformation. Arms Transfers to Non-State Actors follows changes in the prohibition of arms transfers to non-state actors since the pivotal International Court of Justice's Nicaragua ruling in 1986. Hannah Kiel critically discusses the legal developments through relevant case studies, including Abkhazia, Bosnia, Congo, Eastern Ukraine, Kosovo, Libya, Northern Iraq, South Ossetia, Syria and Yemen. Adopting a customary law perspective while also placing the narratives of states in the context of international structural changes, Kiel emphasises the interplay between state practice and the strengthening of a human rights-based paradigm. Kiel ultimately shows that changes in norms at the individual level indicate a larger transformation in the international order, and while the arming of non-state actors remains formally illegal, the prohibition of this practice is informally eroding. Interdisciplinary in scope, this book provides valuable insights for scholars and researchers of public international law, human rights, international humanitarian law, and international relations. It is also of great benefit to human rights lawyers, policymakers, and diplomats.