The development of microdamage under the deformation conditions of high temperature creep, cold metal forming, superplastic forming, and hot metal forming has been reviewed and discussed, and typical constitutive equations developed to model the individual damage mechanisms are summarized. Based on the microstructural analysis of the key damage features for metallic materials under a wide range of deformation conditions, a set of schematic diagrams is designed to illustrate the major types of damage mechanisms. This helps researchers and engineers to understand the major cause of failure of materials under different deformation conditions and to select simple and appropriate mechanism-based damage equations to predict the damage evolution. Further discussions are carried out on the dominant damage mechanisms in hot metal forming conditions and it is concluded that the dominant damage mechanism can be 'grain boundary (creep-type) damage' or 'plasticity-induced (ductile) damage' depending on the material microstructure and deformation rate. In the case of grain boundary damage in hot forming, the shape of microdefects is different from those in high temperature creep and superplastic forming although all of those result in intergranular failure of materials. Furthermore, damage calibration techniques for different conditions of plastic deformation are summarized and discussed.