The Return on Investment Factsheet explains return on investment (ROI) and how it is used; the key ROI findings from the impact study of the Economic and Social Data Service in the UK; the evidence we have that data collections are usually appreciating assets; and how to consider “the cost of inaction” and the counter-factual position if no archive exists. It is intended to be used with other components of the CESSDA Saw Cost-Benefit Advocacy Toolkit to help you understand the approaches and tools available and to support your advocacy to funders and policymakers.