These materials were designed to be used by life science students for instruction in the application of physical theory to ecosystem operation. Most modules contain computer programs which are built around a particular application of a physical process. This module describes the application of irreversible thermodynamics to biology. It begins with explanations of basic concepts such as energy, enthalpy, entropy, and thermodynamic processes and variables. The First Principle of Thermodynamics is reviewed and an extensive treatment of the Second Principle of Thermodynamics is used to demonstrate the basic differences between reversible and irreversible processes. A set of theoretical and philosophical questions is discussed. This last section uses irreversible thermodynamics in exploring a scientific definition of life, as well as for the study of evolution and the origin of life. The reader is assumed to be familiar with elementary classical thermodynamics and differential calculus. Examples and problems throughout the module illustrate applications to the biosciences. (Author/CS)