Publisher Summary This chapter discusses and analyzes different types of viscometers. Only viscometers giving the viscosity as the shear stress in a liquid divided by the shear strain rate for Newtonian liquid behavior is described in the chapter. Behind the analysis of all the different types of apparatus discussed in this chapter is the fundamental assumption postulated by Newton. The shear stress in a liquid is proportional to the shear strain rate, and the constant of proportionality η is a property of the liquid, which is only a function of pressure and temperature, and independent of the shear strain rate. This is only true for rather low shear stresses in the liquid, but can be used to calculate oil film thicknesses with good accuracy. However, the traction forces acting in a lubricant film at high pressure cannot be predicted using a Newtonian model. Different types of falling body viscometers measure the viscosity of fluids. The principle is that a solid body, having a higher density than the liquid to be tested, slowly falls through a liquid-filled tube. The density difference and the gap between the falling body and the tube wall determine the velocity of the falling body.