This paper examines how Save the Children--an international non-governmental organisation (INGO)--assesses the changes it has brought about in children's lives around the globe. The paper begins with a discussion of the role of INGOs. It moves on to examine how we know we are making a difference and gives an explanation of "Global Impact Monitoring"--the framework that Save the Children uses to plan and assess its impact as well as to determine how this impact came about, or to put it another way, to determine "best practice". The paper explains that Save the Children has chosen this system because it reflects a rights-based approach to development and encourages a culture of reflection and learning. The rest of the paper discusses factors that help and hinder the organisation's ability to assess its impact, with practical examples given throughout. The discussion covers a variety of topics, including: how to involve stakeholders (those directly involved in, and those that have an interest in our work, including staff, governments, communities, and most importantly, children and young people) in measuring and analysing what changes Save the Children has created and how these changes came about; how to build monitoring and data analysis into the programme planning cycle; how to ensure sufficient time and resources are allocated for data collection and analysis; how to measure things that are hard, if not impossible, to measure; and, how to measure change at a global level. The paper finishes with a discussion of how to improve things in the future, concentrating on how to build a learning culture within organisations with sufficient resources and incentives to change the way we think about, and practise, development. (Contains 9 notes.)