This analytic review focuses on theory and research on volunteerism. First, we define volunteerism as freely chosen helping activities that extend over time and that are often performed through organizations and on behalf of receptive causes or individuals. Next, we link these definitional features to the Volunteer Process Model, which depicts volunteerism as a process with three sequential and interactive stages (antecedents, experiences, and consequences) and at multiple levels of analysis. Then, we use this model to organize the empirical literature on volunteerism and selected work on social movements. Finally, we discuss implications for social policy issues relevant to individuals, organizations, communities, and societies.