Tackling defence corruption is about much more than stopping dodgy commissions on arms sales. Civilians and soldiers die because of corruption in defence, insecurity rises, peace is compromised, wars extended. This working paper describes the twenty-year effort by Transparency International’s Defence & Security Programme to reduce defence corruption across the whole sector, collaborating with and pressuring defence ministries, military forces, defence companies, NATO, NGOs and others. The progress suggests that a ‘whole sector’ approach – covering companies, governments, regulators and civil society – can be effective, and may hold lessons for how reform approaches in other sectors can be better conceived and implemented.