The notion of a living being is one of the basic concepts of our everyday ontology. Biology may be the science of living beings, but it is not the science of what it is to be a living being. Answering this question is the business of metaphysics. The author undertakes a thorough study of the ontological status of living beings. She opposes the Cartesian idea of living beings as bodies, arguing instead for a view which is based on the Aristotelian concept of substance. For this she first defends the general category of continuants (enduring entities) against objections of process-ontologists. She then goes on to show why living beings cannot be categorized as things, but form a separate category of continuants.