Amorphous solid dispersion is an important formulation technology to improve oral absorption of poorly soluble drugs, where crystallization tendency of the active pharmaceutical ingredient (API) must be comprehended well. Crystallization tendency of API can be estimated by observing crystallization during cooling/reheating of the melt. Though the crystallization during cooling/reheating cannot be directly correlated with crystallization tendency during isothermal storage, which is of great interest in pharmaceutical industry, it provides many insights regarding crystallization mechanism. Initiation time for isothermal crystallization can be explained only by a ratio of glass transition and storage temperatures under well-defined conditions, where knowledge on crystallization tendency is important for expecting other influential factors such as surface area. In this article, assessment of crystallization tendency of pharmaceutical glasses and its relevance to pharmaceutical developmental study is introduced.