This chapter presents the principal features and activities in a straightforward and unified manner. It outlines three aspects where the Fabry–Perot interferometer has contributed significantly. These are: successive comparisons of wavelength and metre bar standards, a comparison of wavelengths initially of krypton lines and then laser lines, and other contributions to metrology. Fabry–Perot interferometers have been used to establish various standards via fundamental length measurements. Terrien used an interferometric manometer for measurements of pressure. The chapter reviews four principle applications of Fabry–Perot interferometers at sub-millimetre wavelengths: with far-infrared lasers; in astronomy; plasma diagnostics and other applications, notably dielectric, absorption and refractive index measurements. Suggested industrial applications include quality control and material testing of for example the moisture content in paper strip drawn through a cavity operating in the 4 mm waveband. Fabry–Perot sensors of great sensitivity have been widely applied and include conventional cavities, solid etalons and fibre-optic resonators.