This chapter reviews literature on the motivation and impact of nature and heritage volunteers, regardless of sector. The focus is on volunteers engaged in serious leisure activities, or acts of impure altruism, seeking benefit both for self and others, including the natural world. A functional model of volunteerism in environmental causes is introduced, along with other models that can guide research in this area. Impact is addressed as the challenging issue that it is, as organizations tend to focus on inputs of volunteerism, such as counting volunteer hours, rather than on impact. Based on literature that spans nonprofit and public sectors, the state of impact measurement is addressed.