Health geography is concerned with the collection, analysis and presentation of data that measures or represents the health of individuals or populations in light of geographical factors. This chapter explores the ongoing role of quantitative data in health geography in light of developments sometimes termed the data revolution. It defines what quantitative data are, describes how to distinguish between different types of quantitative data, and suggests some questions relevant to working with any dataset. The chapter describes the changing nature of data and focuses on examples in health geography that illuminate big data, open and volunteered data and then data infrastructures and the value of indexical data. For quantitative health geographers, evidence for social justice relies on quantitative data about people's personal and environmental circumstances. A solid appreciation of data is fundamental to making meaningful contributions to health geography and social justice.