In this brief monograph, the author derives a nonlinear theory of thermoelasticity from purely phenomenological considerations. Approximations to this theory lead to homogeneous and dissipationless thermoelasticity and linearized thermoelasticity. The aim of the author is to utilize the mathematical formalism of these field theories to gain insight into some of the fundamental properties of thermodynamics: in particular, the concepts of quasistatic, reversible, and irreversible processes.