A variety of viscometers are used in the polymers and plastics industries. These devices fall into two general categories, namely, scientific instruments designed to make basic measurements of rheological properties and instruments that are primarily employed for industrial quality control. This chapter outlines the operating principles and basis for both general classes of instruments. It should be noted that a wide variety of rheological test methods and instruments are used in other industries. A summary of the major rheological measurement techniques for different industries is given in the last section of this chapter.