The demand for return in investment (ROI) analyses in public health is on the rise as a ‘business case’ needs to be presented before a public health programme can be funded. This book explains the science behind ROI analyses and shows how policy makers can learn the art of understanding and using it in the context of public health. Using examples drawn from the NICE ROI Tools, business cases for various alternative policy options around smoking cessation, improving physical activity and reducing alcohol use are presented. In addition, several examples of real-world decision problems facing public health investors are discussed to address the usefulness of ROI evidence in public health policy making.