This paper examines how volunteering varies over the life course. Based on three theoretical explanations (resources, interests, and role substitution), we analyze how changing family characteristics, employment status, and educational attainment affects individual volunteering behavior. Drawing on longitudinal data from the German Socio-Economic Panel (SOEP, 1985&-2009), we compare estimates from between-effects and fixed-effects models. In this way, we discriminate between variation in volunteering frequency that is due to differences between social groups and changes over time, respectively. We find that volunteering behavior is relatively stable over the life course. Moreover, some of the differences that we observe between individuals are no longer statistically significant once we focus on within-person variation. This finding shows the importance of unobserved heterogeneity and selection into volunteerism, which has not been addressed systematically in previous work. Although life-course events have an impact on the frequency of volunteering, their influence is limited and largely constrained to events occurring in the family domain.