Institutional units within the Private non-profit sector historically have played a significant role in R&D activities in many countries. Non-profit institutions (NPIs) can be identified and classified in all sectors; they can be either market producers or non-market producers; and they include both performers and funders of R&D. This chapter outlines which NPIs should be considered for measurement in the PNP sector, and it gives guidelines on the measurement of their R&D activities, taking into account their particular characteristics as well as emerging trends with respect to new forms of R&D funding. The sector is residual, in that NPIs not classified to business enterprise, government or higher education are classified to the Private non-profit sector. The sector also includes, for completeness, households and private individuals engaged or not in market activities. The chapter provides guidance on the institutional classifications by main economic activity, the measurement of R&D expenditures and personnel in the sector and there is a discussion of survey design and data collection in the sector. There is also a brief discussion, new to the manual, of the role of philanthropists and of crowdfunding and the implications for measurement.