This Element is an introduction to the metaphysics of biology, a very general account of the nature of the living world. The first part of the Element addresses more traditionally philosophical questions - whether biological systems are reducible to the properties of their physical parts, causation and laws of nature, substantialist and processualist accounts of life, and the nature of biological kinds. The second half will offer an understanding of important biological entities, drawing on the earlier discussions. This division should not be taken too seriously, however: the topics in both parts are deeply interconnected. Although this does not claim to be a scientific work, it does aim to be firmly grounded in our best scientific knowledge; it is an exercise in naturalistic metaphysics. Its most distinctive feature is that argues throughout for a view of living systems as processes rather than things or, in the technical philosophical sense, substances.