This article discusses the role of alloying in the production and use of low-expansion alloys such as iron-nickel (Invar), iron-nickel-chromium (Elinvar), and iron-nickel-cobalt (Super-Invar and Kovar). It explains how the coefficient of thermal expansion varies with nickel content and how it can be tailored, along with other properties, through appropriate alloying adjustments. The article also discusses the effect of alloying on Incoloy and Pyromet, which are classified as high-strength, controlled-expansion alloys.