Two studies examined individual and environmental forces that affect engagement in prosocial behavior. Self-determination theory was used to derive a model in which autonomy orientation and autonomy support predicted satisfaction of three core psychological needs, which in turn led to engagement in prosocial activities. In Study 1, college students reported their engagement in various prosocial activities, and completed measures of autonomy orientation, parental autonomy support, and general need satisfaction. In Study 2, volunteer workers completed measures of autonomy orientation, work autonomy support and need satisfaction at work. The number of volunteered hours indicated the amount of prosocial engagement. Results across the studies showed that autonomy orientation was strongly related to engagement in prosocial behavior, while autonomy support was modestly related. Need satisfaction partially mediated the effect of autonomy orientation, and fully mediated the effect of autonomy support. Interestingly, autonomy support predicted lower volunteer turnover. Implications for how prosocial behavior can be motivated are discussed.