This chapter deals with heavy loaded spur gears operating at severe conditions that have three main sources: (i) instability of center distance and misalignment of axes of pinion and gear due to the variability of load and cantilever mounting of the pinion; (ii) dynamical forces in teeth meshing caused by the manufacturing errors and the surges of external load; (iii) presence of the abrasive particles and water in the lubricant. For these gears even on the early stage of service the operating fluctuation caused by kinematical error of tooth flanks due to their wear is much more intensive than the one caused by the manufacturing errors. The dependence of tooth thickness on the operating life was defined based on the results of periodical tooth thickness measurements. The appropriate predicting procedure with the calculation results is given. Using the profilography method for the tooth flanks the nature of the profile form deviation was defined. The analysis of the profile forms obtained on the different stages of the wear progress shows that the intensiveness of the deviation of tooth flank from the involute on the early stage is not equal for different segments of tooth flank. Later, the form of profile becomes constant and only the tooth thickness changes. Based on the real tooth profiles the fluctuation of gear ratio during the meshing is defined for different degrees of wear taking into account the positive deviation of center distance. Obtained results will allow to define the dynamic component of load which, when taken into account during the laboratory tests on wear resistance, can provide more reliable data that will allow to choose better materials, heat treatment conditions, and finishing processes for heavy loaded spur gears.