This report describes the role of data in early childhood pay for success (PFS) projects, with a focus on early childhood education. As part of a larger toolkit for states, localities, and investors considering early childhood PFS deals, the report provides a roadmap for collecting and using data. It builds on what we know about effective early childhood interventions (see Toolkit Report #1: The State of the Science on Early Childhood Interventions), and its content is based on the experiences of stakeholders involved in ongoing PFS projects. We first discuss the different ways to use data in PFS projects, then present key considerations for collecting and using data. We provide a checklist of the data-related questions that states, localities, and investors should consider. We then describe common data challenges and potential technology solutions to support data integration and analysis. We conclude with lessons shared from ongoing PFS projects, including the importance of engaging data experts from relevant agencies early in the process, maximizing the opportunity to get communities to consider collaborating more wisely around data, and choosing providers with strong data capabilities. (author abstract)