Until recently, the social mission of most Non Governmental Organisations (NGOs) has tended to exempt them from the levels of scrutiny on issues of performance and accountability found in the private or public sectors. The last few years however have seen NGOs working in international development and relief paying increasing attention to demonstrating the impact of their work on those they purport to help. In 2001, Save the Children UK began developing a new framework for assessing the impact of its work in order to improve the organisation’s accountability as well as learn from its work to maximise its impact. This paper documents some of SC UK’s experience in developing the new framework, and some of the lessons we have learned from implementing it in two-thirds of our 60 country programmes. It aims to provide a frank account of the process, its strengths and weaknesses, and draw some conclusions about improving impact assessment processes. It is hoped that our experiences may be of interest to other development actors, many of whom are grappling with similar issues. Section 2 focuses on why development agencies are wrestling with the issue of impact assessment and some of the theoretical challenges they face. Section 3 covers the development of SC UK’s impact assessment framework, and its links to rights-based approaches to development. Section 4 documents lessons learned on both a theoretical and practical level. Sections 5 and 6 conclude by looking at some of the work currently being undertaken to address the lessons learned and some of the issues that remain to be addressed by the development sector as a whole.