How do we know what is possible? Modal Knowledge in Action motivates, develops, and defends a novel action-first approach to answering that question. On this approach, our modal knowledge is for action: its primary function is to enable us to choose what to do. It is also primarily about action: its primary object is an agentive form of modality that comprises our own abilities and the opportunities that the environment offers us for exercising them. And it is primarily from action: our agency itself provides the first and most important source of our modal knowledge. In short, modal knowledge is deeply embedded in action. Modal Knowledge in Action takes this action-first approach seriously, beginning with a closer look at the agentive modal notions of ability and affordance and our knowledge of them. From there, it develops an action-first account of our ordinary knowledge of possibility and necessity, and finally of our less ordinary knowledge of metaphysical modality. Drawing on arguments and methods from within and outside philosophy, the book develops an integrated account of our modal knowledge that shows how an action-first angle can illuminate our modal thought far beyond the context of action itself.