One of the primary tasks of the philosopher is to explain what it is for something to be the case – what it is for one event to cause another, what it is for an action to be obligatory, what it is for an object to bear a property, what it is for a proposition to be true necessarily, what it is for a person to know something. This activity of explaining what something is or what it is for something (...) to be the case, of identifying what I call ontological explanations, is of special importance to metaphysics, since the task of metaphysics generally is to get to the bottom of reality. The concept of ontological explanation is usually buried a layer deep in most discussions, however, and theses about it are either presupposed or clothed as claims about other things. In some cases, this leads to confusion and frustration, and in many other cases the discussion could benefit from a long look at ontological explanation even if that look isn't strictly necessary to remedy confusion. My goal is to give ontological explanation that long look, and then use the clarity gained to reinterpret, reorganize, and even make progress on some long-standing disputes in metaphysics. In the first two chapters I examine ontological explanation itself and connect it to a host of important metaphysical issues, including ontological commitment and truthmaker theory. In the third and fourth chapters, I apply the work done in the first two chapters to a pair of important metaphysical arguments that crucially employ infinite regresses of ontological explanations – Bradley's Regress and McTaggart's Paradox. (shrink)