One of the major rationales for conducting health promotion/disease prevention activities at the worksite is the potential to reach a high percentage of employees, including many who would otherwise be unlikely to engage in preventive health behaviors. Most studies of worksite health promotion do not report participation data, but among those that do, definitions of participation and participation rates vary dramatically. In general, men and blue-collar employees appear less likely to participate, but little is known about other employee variables related to participation. There have been few studies of worksite characteristics associated with participation and even fewer experimental evaluations of interventions to increase participation. In this paper we review what is known about participation in worksite health promotion programs and recommend procedures for defining participation for different types of programs, for reporting determinants of participation, and for increasing participation. We conclude that participation is both an important process measure and an outcome that should be reported routinely. Participation data have important implications for generalizability of results, feasibility of interventions, and health outcomes.