Interferometry is the technique of superimposing (interfering) two or more waves, to detect differences between them. Interferometry is applied in a wide variety of fields, including astronomy, fiber optics, optical metrology, oceanography, seismology, quantum mechanics and plasma physics.

Interferometry works because two waves with the same frequency that have the same phase will add to each other while two waves that have opposite phase will subtract. Typically, in an interferometer, a wave is split into two (or more) coherent parts, which travel different paths, and the parts are then combined to create interference. When the paths differ by an even number of half-wavelengths, the superposed waves are in phase and interfere constructively, increasing the amplitude of the output wave. When they differ by an odd number of half-wavelengths, the combined waves are 180° out of phase and interfere destructively, decreasing the amplitude of the output. Thus anything that changes the phase of one of the beams by only 180°, shifts the interference from a maximum to a minimum. This makes interferometers sensitive measuring instruments for anything that changes the phase of a wave, such as path length or refractive index.

Early interferometers principally used white light sources (e.g., Young's double slit experiment of 1805). Modern researchers often use monochromatic light sources like lasers, and even the wave character of matter can be exploited to build interferometers (e.g. with electrons, neutrons, atoms, or even molecules).