Abstract This chapter contains the engineering basis for renewable energy conversion into desired energy supply forms. After introducing the general transformation theory for mechanical and thermodynamic processes, the treatment of specific heat and electricity conversion systems are described. Engines, heat pumps, boilers, furnaces, a range of flow-driven converters, photovoltaic and photothermal devices, coolers, electrochemical converters such as fuel cells, and tools for biological conversion into heat or power.