Publisher Summary This chapter discusses basic principles of energy conversion and outlines engineering design details for specific energy conversion devices, ordered according to the energy form being converted and the energy form obtained. In several cases, more than one energy form emerges as a result of the action of the device such as heat in addition to one of the other energy forms. Many devices also perform a number of energy conversion steps, rather than the single ones such as a power plant may perform the conversion process chain between energy forms such as first step involves the conversion of chemical energy to heat energy, which in turn gets converted to mechanical energy that gets converted to electrical energy. Diagonal transformations are also possible, such as conversion of mechanical energy into mechanical energy or of heat into heat at a lower temperature. The second law of thermodynamics forbids a process in which the only change is that heat is transferred from a lower to a higher temperature. Such transfer can be established if at the same time some high-quality energy is degraded such as by a heat pump. The efficiency with which a given conversion process can be carried out depends on the physical and chemical laws governing the process. The principle of an energy conversion device involves a mass flow into the device and another one out from it, as well as an incoming and outgoing heat flow. The work output may be in the form of electric or rotating shaft power. It may be assumed that the converter is in a steady state, implying that the incoming and outgoing mass flows are identical and entropy of the device is constant.