Maraging steels are high-alloyed steels with high strength, good ductility and a good corrosion resistance. Because of their combination of high strength and good toughness, the application areas of these steels are the aircraft industry and the are also used for plastic mould. These kinds of steels are low carbon steels, which obtain their high strength by the formation of intermetallic precipitates in an iron-nickel-matrix. The formation of so-called reverted austenite leads to an improved toughness. To get an optimal ratio between strength and toughness it is important to know the processes during heat treatments which control these properties. In this work the formation process, the morphology, the chemical composition and the orientation of reverted austenite are investigated. To specify the controlling process of the formation of reverted austenite, its activation energy was determined. For the examination of the morphology, the chemical composition and the orientation relationship transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) were used. Another part of this work was to determine the amount of retained austenite on solution heat treatment temperature and time to be able to select proper heat treatments for industrial applications.