The role and approach of NGOs in development has changed radically over the last 20 years. NGOs are now accepted as significant contributors to the development process by governments and official agencies. In the early 1980s it was assumed that the NGOs would have an impact because of who they were and their relationship and closeness to the ‘beneficiaries’. This unsubstantiated assumption has increasingly come into question; throughout the 1990s the issue of assessing NGO impact and the need for appropriate methodologies to do so came to the fore. As the profile of NGOs has increased, so too has the need for them to assess the impact of their work. The rise in popularity of NGOs and the increase in funding channeled through them by governments has had consequences in terms of performance and accountability. In addition, NGOs have become more critically aware themselves of the need to assess their impact, both for organisational learning and strategy development and in order to inform an increasingly discerning public supporter base. This Policy Briefing Paper explores the current state of the debate on impact assessment of development interventions. It moves on to review the current state of practice and methodologies and concludes with a summary of critical issues and implications for NGOs.