NATO is facing major changes and challenges: enlargement, new threats, new missions, new technology, and declining defence budgets. These developments raise the question of who will pay for the changes and hence the possibility of new burden-sharing debates. burden-sharing was a focus of controversy in the past and it could re-emerge in the future. A variety of burden-sharing measures are reviewed. These range from such traditional indicators as the share of defence in GDP to a range of alternative military measures as well as civil indicators, such as contributions to UN humanitarian operations and economic aid. Burden-sharing debates are affected by choice of indicator. Different indicators give different rankings and results. Nations will select the indicator(s) which show that they are bearing an 'unfairly' high burden of the collective defence effort. The final part of the article examines the likely developments in burden-sharing over the next decade (e.g. new missions, new technology, enlargement). On enlargement, emphasis is placed on the need to assess both the benefits and costs of NATO expansion and the conclusion focuses on the optimal size of NATO.