How do we know what is possible or impossible, what is inevitable or unattainable, or what would happen under which circumstances? Since modal facts seem distinctively mysterious and difficult to know, the epistemology of modality has historically been fraught with uncertainty and disagreement. The recent literature has been dominated by rationalist approaches that emphasise a priori reasoning (sometimes including direct intuition of possibility). Only recently have alternative approaches emerged which recognize a broader range of sources of modal knowledge. Yet even emerging non-rationalist views have tended to assign scientific investigation at best a supporting role. Our project in this book is to develop and defend a new approach to the epistemology of modal facts which assigns a central role to scientific investigation. According to modal naturalism, science (construed broadly) is our primary source of evidence concerning the modal facts.