Publisher Summary This chapter discusses the history of rheology and presents the most recent research results regarding the influence of microscopically small highly stressed areas in lubricated machine elements. It provides methods to measure rheological properties of the lubricant, both Newtonian and non-Newtonian and the limits of Newtonian behavior caused by both high stresses and by elasticity at low stresses. These lubricant properties were then used to calculate the oil film thickness numerically. The chapter provides an overview of how people looked upon liquid lubricants and their properties. The definition of Newtonian behavior is that the shear stress in a liquid increases linearly with the shear rate. The ratio between the shear stress and the shear rate is called “the viscosity of the liquid.” For a Newtonian liquid, the viscosity is not a function of the shear rate but can be a function of pressure and temperature. The rheological models used for non-Newtonian liquids are based on curve fitting techniques where experimental measurements of shear stress-shear strain rate relationships are approximated by mathematical expressions.