We are responsible for some things but not for others. In this thesis, I investigate what it takes for an entity to be responsible for something. This question has two components: agents and actions. I argue for a permissive view about agents. Entities such as groups or artificially intelligent systems may be agents in the sense required for responsibility. With respect to actions, I argue for a causal view. The relation in virtue of which agents are responsible for actions is a causal one. I claim that responsibility requires causation and I develop a causal account of agency. This account is particularly apt for addressing the relationship between agency and moral responsibility and sheds light on the causal foundations of moral responsibility.