It has been more than two decades since the publication of the United Nations Handbook on the System of National Accounts (Salamon & Anheier, 1994; Einarsson & Wijkström, 2019). This international standard setting approach for data collection, measurement, and reporting of national data has been joined by others including the United Nations, World Bank, OECD, and Open Government Partnership. Collectively these international data projects have increasingly improved their recognition and measurement of broad sets of third sector organizations, philanthropy, and volunteer work and provided important opportunities to produce foundational comparative data that bring new visibility and credibility to the third sector as well as enabling new research. This chapter provides an overview of current data aggregation efforts as well as the key issues that any data aggregation project must consider, including data quality, scope, commensurability, and durability.