Much recent work on thermodynamic energy conversion efficiencies is here presentedabinitioand from a unified point of view. New considerations are offered and previous work is analyzed in light of the unified presentation. The energy conversion processes discussed include, for example, the conversion into light in a laser or light-emitting diode and the direct or indirect conversion of solar radiation into work. The thermodynamic limiting efficiencies of these various processes are derived by applying simple balance equations to a generalized energy convertor. The new work includes the distinction between temperatures based on fluxes and temperatures ∂U/∂Sbased on standard thermodynamics, etc., and their application in the correct context. Also the use of standard entropy formulas is justified without relying on thermodynamic equilibrium and the results are then applied to nonequilibrium situations in energy conversion.