This guide sets out the issues that need to be considered when developing a business case to invest in preventive services and to ensure that any decision are based on robust and reliable data. The guide focuses on the following arguments: the importance of ‘investing to save’, arguing that prevention is cheaper in the long term; promotion of service innovation; placing the focus of commissioning on outcomes rather than outputs; and managing a shift in spending from acute to prevention to reduce demand over time. The guide outlines key four activities required to build a business case: understanding needs; understanding current costs; assessing possible interventions; and deciding how to measure the value and outcome of the interventions. It also provides a summary business case for prevention and using a Social Impact Bond (SBI) to finance a business case for prevention. An example case study of making a business case for prevention services in early years services in Greater Manchester is included.