Living systems are complex systems made of components that tend to degrade, but nonetheless they maintain themselves far from equilibrium. This requires living systems to extract energy and materials from the environment and use them to build and repair their parts by regulating their activities based on their internal and external conditions in ways that allow them to keep living. The philosophical and theoretical approach discussed in this Element aims to explain these features of biological systems by appealing to their organization. It addresses classical and more recent issues in philosophy of biology, fromorigins and definitions of life to biological teleology and functions, from an original perspective mainly focused on the living system, its physiology and behavior, rather than evolution. It discusses and revises the conceptual foundations of this approach and presents an updated version of it. This title is also available as Open Access on Cambridge Core.