Research interest in the thermo-mechanical behaviour of soils is growing as a result of an increasing number of geomechanical problems involving thermal effects. This paper addresses various issues concerning the thermo-mechanical behaviour of soils. Starting from experimental evidence of non-linearity and hardening that heating causes in clayey soils, a general thermo-plastic formulation is then introduced for the derivation of the constitutive equations. After that, constitutive modelling is presented and treated in the context of elasto-plasticity. Numerical simulations support the theoretical aspects of the paper by showing performances of the constitutive model.