This commentary on thermodynamics in solid mechanics aims to provide an overview of the main concepts of thermodynamic processes as they apply to, and may be exploited for, studies in nonlinear solid mechanics. We give a descriptive commentary on the (physical) interpretation of these concepts, and relate these where appropriate to behaviour of solids under thermo-mechanical conditions. The motivation is firstly that students of solid mechanics have often had less exposure to thermodynamics than those in other branches of science and engineering, yet there is great value in analytical formulations of material behaviour derived from the principles of thermodynamics. It also sets the contributions in this Theme Issue in context. Along with the deliberately descriptive treatment of thermodynamics, we do outline the main mathematical statements that define the subject, knowing that full details are provided by the authors in their corresponding contributions to this issue. The commentary ends on a lighter note. In order to aid understanding and to stimulate discussion of thermodynamics in solid mechanics, we have invented a number of very basic and completely fictitious materials. These have strange and extreme behaviours that describe certain thermodynamics concepts, such as entropy, in isolation from the complexities of real material behaviour.