After a brief historical introduction, this paper proceeds to derive expressions for the available work of a thermodynamic system while the latter interacts with the passive environment. The passive environment is modelled as a union of three idealized reservoirs — energy, volume, and matter. The available work is defined as the optimum amount — largest or smallest — exchanged in circumstances given a prescribed initial and final state of the system. Two types of systems are studied — closed systems and open flow systems — and the interaction is progressively enlarged to include the transfer of heat, the transfer of work, and the transfer of matter. Available work is the difference between the exergy of states 1 and 2; the exergy of a state being defined as the optimum work achieveable against a final state which is one of equilibrium with the surroundings. It is shown that exergy so defined is not a unique quantity for a given system and state, because it also depends on the kind of interaction permitted and therefore on the kind of equilibrium achieved. Exergy, and not energy, is found to represent the preferred measure of a system's “capacity to perform work”. Remarks are made about the contemporary terminology of this field, and regrets are expressed concerning the present day proliferation of terms.