This article describes the general categories and metallurgy of heat treatable aluminum alloys. It briefly reviews the key impurities and each of the principal alloying elements in aluminum alloys, namely, copper, magnesium, manganese, silicon, zinc, iron, lithium, titanium, boron, zirconium, chromium, vanadium, scandium, nickel, tin, and bismuth. The article discusses the secondary phases in aluminum alloys, namely, nonmetallic inclusions, porosity, primary particles, constituent particles, dispersoids, precipitates, grain and dislocation structure, and crystallographic texture. It also discusses the mechanisms used for strengthening aluminum alloys, including solid-solution hardening, grain-size strengthening, work or strain hardening, and precipitation hardening. The process of precipitation...