There could be no more obvious a truism than that biology is the study of living things—organisms. And yet, there is a strong sense in which modern evolutionary biology is not about organisms at all. Our current best theory of evolution deals in supra- organismal assemblages (populations) of sub-organismal entities (genes or replicators). Indeed, in the 150 years since the Origin of Species biology has been marked by two related trends, the dramatic growth in the power and breadth of evolutionary thinking and the marginalization of organisms. This latter trend has been celebrated by some, lamented by a few, but, remarkably, overlooked by most. The reasons for the loss of organisms are varied and complex; they are empirical, historical and philosophical. This book explores them and it outlines an alternative conception of evolutionary biology in which organisms play an ineliminable explanatory role. These issues are only now beginning to attract interest in both evolutionary biology and philosophy. So this book will constitute a timely introduction to a vibrant, growing debate. While the book’s objective is specific, its brief is broad. It draws from the history of organismal biology from ancient to modern times, empirical issues in evolutionary biology, population genetics and developmental biology and systems theory. It deploys these empirical considerations in addressing the philosophical foundations of evolutionary biology. The book will appeal to philosophers of biology and philosophers of science in general, as well as to evolutionary biologists. Every effort will be made to make the difficult technical material accessible to students of both the sciences and the humanities.