This is the first guidance note in a four-part series of notes produced by InterAction to support management, program and M&E staff in international NGOs to plan, design, manage, conduct and use impact evaluations. This first guidance note, Introduction to Impact Evaluation, provides an overview of impact evaluation, explaining how impact evaluation differs from – and complements – other types of evaluation, why impact evaluation should be done, when and by whom. It describes different methods, approaches and designs that can be used for the different aspects of impact evaluation. The guidance note is also available in French, Arabic and Spanish on https://www.interaction.org/impact-evaluation-notes.