The application of deformation calorimetry to the study of polymer structure and properties has appeared to be rather successful. However, up until now, most of the theories and experiments have dealt with mainly uniaxial stretching. Nevertheless, study of the simple shear deformation mode is valuable both for theory and applications. The theoretical thermodynamic description of simple shear deformation for polymer rubbers and glasses is presented. The application of the results obtained to the investigation of adhesion and fibre/matrix interphase phenomena for, respectively, polymer adhesive interlayers and composite materials is considered.