We investigate positive effects of volunteering on psychological well-being and self-reported health using all four waves of the Wisconsin Longitudinal Study. Confirming previous research, volunteering was positively related to both outcome variables. Both consistency of volunteering over time and diversity of participation are significantly related to well-being and self-reported health. The relationship of volunteering to psychological well-being was moderated by level of social integration, such that those who were less well integrated benefited the most. Mattering appears to mediate the link between volunteering and well-being. Controls for other forms of social participation and for the predictors of volunteering are employed in analyses of well-being in 1992. We find volunteering effects on psychological well-being in 2004, controlling for 1992 well-being, thus providing strong evidence for a causal effect. (Contains 10 notes and 7 tables.)