Publisher Summary This chapter discusses the study of irreversible thermodynamics. The study of thermodynamics begins in the proper sense, by exploring a variety of physical situations in a system in which one or more variables are rendered nonuniform. This result in the establishment of irreversible phenomena is explored. If the variations in “T”, “P”, “ρ” or other intensive quantities are ‘small’ relative to their average values, it is acceptable to apply the machinery of equilibrium thermodynamics in the manner discussed. The identification of conjugate forces and fluxes, the Onsager reciprocity conditions, and the rate of entropy production play a central role in the subsequent analysis. Any non-equilibrium phenomena necessarily involve states in which different portions of a given system display different physical characteristics. In order to handle this situation, the system is subdivided into tiny subunits and allows all variables of interest to become functions of their position within the sample, and also to become functions of time.