On the basis of a physical picture of a glass as a mosaic of mesoscopic clusters differing in their yield characteristics, we propose a model for the preyield behavior in glassy materials; the model describes the stress- strain relationship at different strain rates in terms of one reduced variable. A test using experimental data for polycarbonate materials at different rates and temperatures appears favorable for the model. The model may be used to interpolate and extrapolate limited experimental data, and also provides a practical means to assess dynamic heterogeneities within polymeric glasses. When applied at different temperatures, the model gives insight into the dependence of the excitation energies on temperature ( glassformer fragility) in the glassy state.