Collective-action problems pervade all societies as well as ecological systems used by humans. Substantial evidence has accrued during the last several decades that human actors are able to solve some (but definitely not all) collective-action problems on their own without external rules and enforcement imposed from the outside. In this article, I review some of the structural variables that have been found to affect the likelihood of collective action. Then, I address the need to base future work on collective action on a more general theory of human behavior than has been used to model collective action over the last five decades. In the last section, I discuss how structural variables affect the core relationships of reputation, trust, and reciprocity as these affect levels of cooperation.