Publisher Summary This chapter discusses the fundamental principles of thermodynamics that govern any physical process. Thermodynamics is an overarching discipline in the sense that all physical phenomena can be described and analyzed in terms of a general macroscopic framework that contains parameters that may be determined by experiment. The fundamental concepts are introduced in the form of four basic laws. The procedure is reasonably axiomatic, so that it can be dealt with the concept of temperature without initially having to refer to heat flow; the definition of energy as a function of state, and the definition of heat flow as a deficit function, the introduction of the entropy function that does not depend on a generalization of the performance characteristics of heat engines. A comparison of the entropy changes for a given process carried out reversibly and irreversibly is then used to obtain a variety of fundamental results. This leads to the introduction of several functions of state; considerable emphasis is placed on systematically exploiting their mathematical properties. The important concept of homogeneous functions of degree one is then introduced and is used to analyze the properties of open systems. The chapter ends with a study of stability problems.