Low-expansion alloys are various binary iron-nickel alloys and several ternary alloys of iron combined with nickel-chromium, nickel-cobalt or cobalt-chromium alloying and are used in applications such as geodetic tape, bimetal strip, glass-to-metal seals, and electronic and radio components. This Article focuses on the properties, composition, and applications of various low-expansion alloys, including iron-nickel alloys (Invar), iron-nickel-chromium alloys (Elinvar), iron-nickel-cobalt alloys (Super-lnvar, Kovar) and iron-cobalt-chromium alloys (special alloys). It further discusses the effect of factors like composition, heat treatment, cold drawing, and humidity on the coefficient of thermal expansion of various low-expansion alloys. The Article provides information on the machining parameters and the physical and mechanical properties of low-expansion alloys. It also provides an outline of special low-expansion alloys, namely, iron-cobalt-chromium alloys, hardenable alloys, and high-strength controlled-expansion alloys.