The presence of public goods raises some fundamental questions about the organization of society and markets as they address questions on the conflict between group and individual interest. While free riding is rational at the individual level, it produces socially undesirable outcomes at the aggregate level. This survey reviews recent work by social scientists intended to isolate fundamental behavioural and motivational aspects in these conflict situations. Two decades of experimental research have revealed that people tend to contribute to public goods at higher levels than theory (Homo oeconomicus) predicts. Going beyond simply demonstrating an "anomaly" (in neoclassical terms) or behavioural phenomenon, a significant number of experimental work has examined the motives that operate in these situations, the contextual variables that influence operating motives and individual differences in the relative strengths and stability of these motives.