The application of classical nucleation theory to the initiation of crystal growth in glasses is discussed. Its application to experimental results obtained for the rate of nucleation in three types of glass, one showing nucleation separately from crystal growth, another showing simultaneous nucleation and growth, and the third showing crystal growth after metastable liquid/liquid phase separation, is then described. Recent work on the kinetics of unmixing in glasses is outlined. The influence of so-called ‘nucleating agents’ in the glasses described appears to be exerted through changes in the anionic structure.