Policy development and rights-based actions depend on the existence of good data and statistics. In the field of disability definition and measurement, the Washington Group on Disability Statistics (WG) has developed internationally comparable questions on disability that can be used worldwide to address both policy development and the rights of persons with disabilities. The WG, a United Nations Statistical Commission sponsored city group, was established to address the methodological and measurement challenges that have characterised disability statistics and to develop questions that will provide cross-nationally comparable, valid and reliable disability statistics. The WG carefully considered the theoretical and conceptual issues associated with disability, transformed disability concepts into measurement tools, and used mixed-method evaluations (both cognitive and field interviewing methodologies) to test the proposed questions. The resultant question sets will greatly improve the international comparability of disability statistics and will aid governments in developing and evaluating policies and programs with the objective of improving the lives of people with disabilities.