This dissertation puts forth an account of moral responsibility. The central claim defended is that an agent's responsibility supervenes on the agent's mental states at the time of the action. I call the mental states that determine responsibility the agent's quality of will (QOW). QOW is taken to concern the agent's action, understood from an internal perspective, along with the agent's motivations, her actual beliefs about the action, and the beliefs she ought to have had about the action. This approach to responsibility has a number of surprising implications. First, blameworthiness can come apart from wrongness, and praiseworthiness from rightness. This is because responsibility is an internal notion and rightness and wrongness are external notions. Furthermore, agents can only be responsible for their QOW. It follows that agents cannot be responsible for the consequences of their actions. I further argue that one's QOW is determined by what one cares about. And the fact that we react to the QOW of others with morally reactive emotions, such as resentment and gratitude, shows that we care about QOW. The reactive attitudes can therefore be understood as ways in which we care about what others care about. Responsibility can be assessed by comparing one's actual QOW to the QOW one ought to have had.