The interferometer is designed to exploit, in any of a variety of ways, the interference of light and the fringe patterns that result from optical path differences. The interferometer is also called an optical interferometer. Applications of interferometers can extend to acoustic and radio waves. This chapter explains the basic principles of the Michelson, Mach—Zehnder, and the Fabr—Perot interferometers. The Michelson interferometer is an amplitude-division interferometer. The Michelson interferometer splits a beam of monochromatic light into two parts, so that one beam strikes a fixed mirror and the other a movable mirror. The original application of the Michelson interferometer was in the historic Michelson—Morley experiment. The Mach—Zehnder interferometer is used in many applications; for example, in aerodynamic research, the geometry of airflow around an object in a wind tunnel is revealed through local variations of pressure and refractive index. The Fizeau interferometer is suitable for metrology application in fields, such as civil engineering, geology, biotechnology, life sciences, and vibration-insensitive field.