Publisher Summary This chapter deals with general principles of energy conversion and covers specific types of conversion suitable for different classes of renewable energy such as wind, hydro and wave energy, solar radiation used for heat or power generation. It also explains the secondary conversions in fuel cell or battery operation, and a range of conversions related to biomass, from traditional combustion to advanced ways of producing liquid or gaseous biofuels. An introduction to all the technical issues to consider in designing renewable energy systems is also presented. The complementary issues of economy, environmental impacts and planning procedures, as well as a basic physical-astronomical explanation of where the renewable energy sources come from and how they are distributed, may be found in the treatise of Sørensen. Some of the renewable energy sources are fundamentally intermittent, and sometimes beyond what can be remedied by regional trade of energy, so the energy storage must also be treated as an important partner to many renewable energy systems.