As conventional energy sources such as fossil fuels become increasingly scarce, scientists and engineers will need to develop more and more new energy sources such as geothermal, solar, ocean thermal, magnetohydrodynamic, nuclear, and hydrogen to meet the world's demands. The utilization of all these forms of energy involves the conversion of heat to mechanical energy. Designed for senior level undergraduates, graduate science and engineering students, this text develops an understanding of the physical processes involved in the transformation of one form of energy to another. The emphasis is on a description of models of the elementary processes, which allows the assessment of performance potential and a determination of the sensitivity to design choices. Since many energy conversion processes involve the manipulation of gaseous substances, there is heavy emphasis on the description of fluids and of gases in particular. Energy conversion processes involve heat and work interactions between a system, its environment, and friction, as well as state and property descriptions. In order to arrive at simple, understandable relations, simplifications are made which allow description at the expense of some numerical accuracy. More accurate descriptions can be made with more sophisticated computational tools and nearly all numerical calculations are done with the equations developed, so that the reader can implement them on a computer and reproduce them with his or her own choice of parameters.