The dissertation develops a conception of action based on the concept of practical task-responsibility rather than on the concept of intention. It answers two problems. First, it grounds the distinction between an action and a mere happening, thus meeting Wittgenstein's challenge to explain what is the difference between an agent's raising her arm and her arm rising? Second, it grounds the distinction between acting for a reason and acting while merely having a reason, thus meeting Davidson's challenge to give an (...) account of the explanatory force of reasons. ;One source of resistance to an account of action in terms of normative expectations rather than in terms of intentions comes from explanatory individualism. The explanatory individualist argues that it is ultimately the intentional attitudes of the agent rather than normative expectations of other people that are relevant to the way in which we explain one another's actions, and that they ought to figure in the account of the nature of action. I defend explanatory nonindividualism, according to which we sometimes act on our own intentions and desires but sometimes on the normative expectations and desires of others . Explanatory nonindividualism is fortified by a selectional account of the explanatory force of reasons. I demonstrate that Davidson's challenge can be met by identifying reasons with selectional criteria rather than with causes. ;In response to Wittgenstein's challenge, I propose that we understand what it is to be an action not in terms of a performance being intentional under a description, but in terms of it being reasonable to expect a performance of the agent under some description. Only one concept of reasonableness is necessary to make the distinction between actions and mere happenings, and I explicate that concept. In addition to accounting for actions that are intentional under some description, the account also captures cases that are not so straightforwardly captured by that criterion . Moreover, cases of so-called basic wayward causal chains are excluded from qualifying as actions. (shrink)