This book provides a concise coverage of the basic concepts of thermodynamics, as it is used to solve the energy, power, and propulsion related issues. Reversible and regenerative principles from thermodynamics, high-pressure combustion processes, fuel cells, and renewable energy sources find application today in mitigating waste heat. The modeling can be used for prediction of cyclones, tornadoes, and other phenomena taking place in nature. It also derives the mathematical expressions about reversibility, work, and equilibrium from physical and intuitive considerations. The treatment is not restricted to ideal gases, and ideal gas assumption is imposed as a particular case of a real gas. Reversible paths between equilibrium states are obtained using reversible heat engines and reversible heat pumps to determine the entropy changes and obtain reversible work. The conditions of thermodynamic equilibrium are addressed for different systems. The molecular basis for temperature, internal energy changes, entropy, reversibility, and equilibrium are discussed. The book serves as a reference for undergraduate and graduate students alongside thermodynamics textbooks.