Can only science deliver genuine knowledge about the world and ourselves? Is science our only guide to what exists? Scientism answers both questions with yes. Scientism is increasingly influential in popular scientific literature and intellectual life in general, but philosophers have hitherto largely ignored it. This collection is one of the first to develop and assess scientism as a serious philosophical position. It features twelve new essays by both proponents and critics of scientism. Before scientism can be evaluated, it needs to be clear what it is. Hence, the collection opens with essays that provide an overview of the many different versions of scientism and their mutual interrelations. Next, several card-carrying proponents of scientism make their case, either by developing and arguing directly for their preferred version of scientism or by responding to objections. Then, the floor is given to critics of scientism. It is examined whether scientism is epistemically vicious, whether scientism presents a plausible general epistemological outlook and whether science has limits. The final four essays zoom out and connect scientism to ongoing debates elsewhere in philosophy. What does scientism mean for religious epistemology? What can science tell us about morality and is a scientistic moral epistemology plausible? How is scientism related to physicalism? And is experimental philosophy really a form of scientism tailored to philosophy?