The performance of a jet, rocket, nuclear, ion or electric propulsion system can always be analyzed by four fundamental parameters, i.e., pressure, temperature, density and velocity inside the system. However, instead of using three physical properties of gases (pressure, temperature, and density), three thermodynamic properties of gases (enthalpy, entropy, and internal energy) are often used for convenience of calculations. Therefore, analysis of propulsion problems can be made by the application of the fundamentals of thermodynamics and aerothermodynamics. In this chapter, these fundamentals of thermodynamics and aerothermodynamics which are particularly applicable to propulsion problems will be given briefly but adequately. Extensive treatment or discussion of these subjects should, of course, be referred to the separate textbooks of thermodynamics and aerothermodynamics. The performance of a jet, rocket, nuclear, ion or electric propulsion system can always be analyzed by four fundamental parameters, i.e., pressure, temperature, density and velocity inside the system. However, instead of using three physical properties of gases (pressure, temperature, and density) , three thermodynamic properties of gases (enthalpy, entropy, and internal energy) are often used for convenience of calculations. Therefore, analysis of propulSion problems can be made by the application of the fundamentals of thermodynamics and aerothermodynamics.