Impact measurement is a crucial component of social finance. This chapter presents a practical overview of the key elements of impact measurement for social finance, including definitions, essential practices, and an introduction to the different frameworks and standards. It also presents some tensions and challenges with impact measurement. The chapter focuses on the tension between the different motivations for undertaking impact measurement—to prove and to improve. This challenge arises because tension exists between uniform metrics, which are well-suited to benchmarking and auditing, and flexible, bespoke approaches, which are well-suited to learning. The challenges discussed are data interoperability and establishing attribution and causality, emphasizing the importance of stakeholder voice and impact valuation.