Maraging steels are highly alloyed low-carbon iron-nickel martensite steels that possess an excellent combination of strength and toughness superior to that of most carbon-hardened steels. This article provides a detailed account of the formation of martensite in maraging steels. It discusses the heat treatment of these steels, namely, aging, solution annealing, age hardening, and nitriding. Their hardening during aging has been attributed to two different mechanisms: short-range ordering and precipitation. The article concludes with a discussion on the grain refinement using thermal cycling and transformation-induced plasticity maraging methods.