In the first part@ this document describes all the types and aspects of wear which can occur on the gear teeth in operation@ with causes and explanations concerning their appearance. For each type of wear@ the possible evolution and the limits which should not be passed to avoid a failure or severe damage to the tooth@ thus reducing the life of the gear@ are clearly specified. Aspects of wear types which can be considered as normal@ medium or progressive are also described and solutions are presented as a guide for wear stabilizing.