 A treatment of thermodynamics naturally begins with the statements it takes to be laws of nature. These laws are founded upon observations of relationships between particular macroscopic parameters and they are justified by the fact they are empirically adequate. No further justification of these laws is to be found—at this stage—from the details of microphysics. Rather, stable, counterfactual-supporting generalizations about macroscopic features are enshrined as law. The typical textbook treatment of thermodynamics describes some basic concepts, states the laws in a more or less rough way and then proceeds to derive the concepts of temperature and entropy and the various thermodynamic equations of state. It is worth remarking, however, that in the last fifty years the subject has been presented with a degree of mathematical rigor not previously achieved. Originating from the early axiomatization by Carathéodory in 1909, the development of “rational thermodynamics” has clarified the concepts and logic of classical thermodynamics to a degree not generally appreciated. There now exist many quite different, mathematically exact approaches to thermodynamics, each starting with different primitive kinds and/or observational regularities as axioms. (For a popular presentation of a recent axiomatization, see Lieb and Yngvason 2000.)