The widely agreed view that actions are events faces the problem of how to describe the “branches” in so-called action trees, i.e. actions which are done by doing other actions. Moreover, the view is also inconsistent with the existence of two familiar species of agency: omitting something and letting things happen. In this article, an alternative conception of action is proposed which takes letting happen as the paradigm of agency. Agency should be construed as an explanatory relation between agents and (...) things happening in the world. This relational view of agency can accommodate for all kinds of agency: doing, letting happen, omitting, prohibiting. And it also provides a satisfactory account of the ontological basis of action trees. We should at first say that to do something is to originate or to bring into existence, i.e., really, to cause, some not yet existing state either of ourselves or of someone else, or, again, of some body. (shrink)