Though the basic phenomenon of wear on gear tooth contact surfaces is the removal of a piece of material from the working surface, the sizes of the wear debris may be different, due to the different physical causes in their formation processes. No common definitions have been established to distinguish spalling from pitting in the literature. This is probably due to the fact that the physical causes of pitting and spalling have not yet been established. In this paper, a brief literature review is presented with the intention to differentiate spalling from pitting. Three types of wear phenomena are defined. Furthermore, the results of a recent experimental study of gear tooth spalling formation in AISI 4340 gears in a test rig are also presented to demonstrate a possible process of spalling due to the development of cracks beneath the tooth contact surfaces and crack linkages in plastically-collapsed metal ligament between the crack tip and the adjacent tooth contact surface. These experimental results substantiated the ligament collapse spalling mechanism proposed by Ding et al. [Y. Ding, R. Jones, B.T. Kuhnell, Elastic-plastic finite element analysis of spall formation in gears, Wear 197 (1996) 197-205; M. Heems, F. Lagarde, R. Courtel, P. Sorin, C. R. Acad. Sci. 257 (3) (1963) 3293].