The concept of EHL, and simplified equations for calculating the isothermal elastohydrodynamic lubricant film thickness between meshing gear teeth, are presented. The only physical property of the lubricant required is the lubricant parameter (LP), which combines both the viscosity and the viscosity-pressure characteristics. The value of LP can accurately be determined by direct measurement of the oil film thickness in an optical EHL apparatus. Since LP is dependent on temperature, measurements are made at different temperatures and the data presented in the form of charts of LP vs. temperature.The quality of lubrication of gears can be expressed as the ratio of the EHL film thickness to the composite surface roughness of the gear teeth, frequently called the specific film thickness. Data relating the probability of gear distress to the specific film thickness will be reviewed. Lubricant selection for a desired value of the specific film thickness can be made through LP.Under certain operating conditions the EHL film thickness may be significantly less than that calculated from theory. Correction factors are provided for non-isothermal conditions and for lubricant starvation in the EHL contact of meshing gear teeth. Effects of VI-improved and non-Newtonian oils are considered.