No discipline or branch of engineering can be enduring without theoretical principles underlying it or without standing on a firm theoretical foundation. In this regard, two cornerstones of sustainability assessment of any system or process should be the second law of thermodynamics and the notion of externalities of macro-economics.

The inevitable entropy increase attributable to a process of concern manifests itself in exergy dissipation by the process, which can be evaluated by resorting to the available energy balance; this balance results from integrating the first and second laws of thermodynamics. In reality, however, the available energy balance also implicitly embodies the mass conservation law: It entails the detailed accounting of every material species involved in the process in light of the species' matter-energy.

Economic externalities attributable to a process of concern manifest themselves in the process' external costs. These external costs can be evaluated through cost accounting or estimation in conjunction with the process' internal costs, such as the labor and equipment costs, inherent to the process.