This book articulates and defends a novel view about fundamental reality, an alternative to the traditional opposition between monism and pluralism. Collective allism is the view that what's metaphysically fundamental is neither the universe as a whole nor some fragments thereof, but the plurality of all entities, i.e. all entities taken collectively. Using resources from plural logic, the book systematically develops the view and argues that considerations about mereological infinitism, emergence, natural laws, and relations give it an edge over monism and pluralism. This book is therefore of great interest to both scientists and philosophers of science.