This chapter provides guidance on defining, identifying and measuring research and experimental development (R&D) personnel, the people who perform R&D, the highly trained scientists and engineers (researchers), technicians with high levels of technical experience and training, and supporting staff who contribute directly to carrying out R&D projects and activities in R&D-performing statistical units. A distinction is made between internal R&D personnel, those employed by the statistical unit, and external R&D personnel. The activities that characterise R&D personnel are listed. Definitions are provided for R&D personnel, external R&D personnel, researchers, technicians and equivalent staff, other supporting staff, full-time equivalent (FTE) of R&D personnel, and the headcount (HC) of R&D personnel. The role of doctorate and master’s students as R&D personnel is discussed. Examples are provided to assist in making decisions about who are to be included as R&D personnel and who not. The resulting statistics on the number, availability and demographic characteristics of human resources that contribute to R&D activities are used by researchers and by policy makers concerned with the sustainability of the R&D enterprise.