A review is presented of several aspects of crystallization processes in glass and their impact on glass-forming ability. Herein, both isothermal and non-isothermal processes are considered. For isothermal crystallization processes, generalizations of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory are described and the status of certain features of nucleation rate and crystal growth calculations in glasses are given. For non-isothermal crystallization, some features of DTA/DSC analyses and critical cooling rate calculations for glass-formation are discussed, and generalizations of the standard theory for computation of the fraction crystallized are presented.