The concrete world contains a great diversity of phenomena. There are quarks, molecules, galaxies, trees, wombats, persons, nerf balls, nations, and more. The properties those entities have, and the relations in which they stand, are no less varied. But is there an underlying commonality--a single fundamental nature or kind or type that everything shares? That is the question of monism. At least, it is a question of monism, which has intrigued philosophers at least since the pre-Socratics. This volume argues that considerations about monism figure importantly in the contemporary mind-body debate. In short, monism matters. To some philosophers, this monism-matters thesis will seem obviously true. After all, they will say, the contemporary debate is largely about physicalism, which is a version of monism, and its rivalry with dualism. To other philosophers, the monism-matters thesis will seem obviously false, perhaps even quaint. After all, they will say, the contemporary debate has a life of its own and should not be tethered to its origins. And indeed, in that debate, monism is rarely mentioned. Monism Matters challenges both of those reactions. Whether monism matters is a complex issue, with arguments worth considering on both sides.