In 2017, about 30% of all U.S. adults volunteered for a total of 6.9 billion hours. This raises the question, why do so many people volunteer? Extant research has produced highly variable estimates of the effect sizes of various motivating factors, and there has been little to no research on potential moderators (i.e., study-level covariates that might strengthen or weaken the main effect of volunteer motives). We meta-analyzed 61 studies ( N = 38,327) to estimate the effect sizes of six volunteer motivators (Volunteer Functions Inventory [VFI]; Clary et al., 1998) in predicting outcomes (satisfaction, commitment, intention to continue, and frequency). Results demonstrate that all six motivators significantly predicted the three outcome variables ([Formula: see text] ranging from .12 to .44). Values was the strongest predictor by far, based on the largest effect size and a post hoc relative importance analysis. Moderator analyses indicated some differences in effect sizes across gender and student status; there were few differences across geographic location, race-ethnicity, college degree attainment, and employment status. Implications for volunteer managers and organizations on how to best work with volunteers are described.