In engineering practice, thermodynamic, economic and environmental aspects are taken into account either separately or in some limited combination during the different stages of energy system synthesis, design and operation. It would be beneficial, however, to use an approach which mathematically models one or more of these aspects in a unified fashion so that optimization algorithms whether deterministic or non-deterministic could be used to search the solution space of all possible solutions for the synthesis, design and/or operation of a new or an existing system. Thermodynamic, thermoeconomic and environomic approaches have been and are continuing to be developed [1-16] which do just that. The purpose of this article is to discuss the principals characteristics of these approaches. Three applications are presented: i) the thermodynamic modeling and optimization of the time- dependent operation of a batch furnace [13], ii) the environomic and thermoeconomic modeling and optimization of a gas turbine cycle’s design and operation [4, 3] and iii) the thermoeconomic modeling and optimization of the synthesis and time-dependent operation of a gas turbine cogeneration / heat pump facility with thermal storage [11, 12].