Between 10% and 15% of all aid to developing countries (over $6 billion) is channelled by or through Non-Governmental Organisations (NGOs). Although emergency assistance accounts for a significant (and in recent years rising) share of total NGO aid flows, the majority of NGO aid is currently provided for development projects and programmes, which are the focus of this Briefing Paper. It examines the evidence of their impact, discusses methods of assessing that impact and reviews the ways that donors and NGOs approach the issue of impact assessment.