I argue that agents can have duties to cooperate with one another if this increases their combined efficiency and/or efficacy in addressing ongoing collective moral problems. I call these duties cooperative duties of efficiency and efficacy. I focus particularly on collective agents and how agents ought to reason and act in the face of global moral problems. After setting out my account, I argue that a subset of cooperative duties of efficiency and efficacy of collective agents are duties of justice (...) in virtue of the roles these agents have taken up. (shrink)