This paper reviews the progress that has been made in our understanding of the chemical strengthening of glass by ion exchange over nearly five decades of history. Lessons learned are briefly discussed; more importantly, those which are yet to be learned are highlighted. It is recognised that, except for detailed compositional effects, the kinetics of ion interdiffusion process and the chemical strengthening technology are reasonably well understood. However, the science of stress generation and its concurrent relaxation is far from being clear, despite the elegant analogy to thermal stresses invoked by Cooper. The need to understand plasticity of glass network during accommodation of a larger invading ion is emphasised. In turn, the influence of network topology on its yield strength in shear as well as hydrostatic modes is recognised.
