Maraging-steels exhibit high strength and remarkable toughness. This combination makes them an ideal material for highly stressed parts in airplanes, the chemical industry und for tooling applications. Applying heat treatments enables to adjust the microstructure and, therefore, the mechanical properties of materials. The aim of this thesis was to assess the influence of different heat treatments, with special attention on intercritical annealing, on the microstructure and mechanical properties of a PH 13-8 Mo maraging-steel. The microstructure was investigated using scanning electron microscopy, X-ray diffraction, electron backscatter diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy and atom probe tomography. Mechanical properties were determined using hardness measurements, tensile testing, Charpy impact tests and J-integral experiments. The tested specimens were further analyzed by means of fractography. The introduction of an additional intercritical annealing step after solution annealing and before the aging treatment led to a higher amount of reverted austenite at lower aging temperatures. Due to intercritical annealing the martensitic matrix was enriched in Ni which is beneficial for the reverted austenite formation. This reverted austenite was thermally more stable against the martensitic transformation. This stability of the reverted austenite stems from the lower tempering of the martensitic matrix at lower aging temperatures. Such matrix is harder and provides an energetical barrier against the transformation of reverted austenite into martensite. Atom probe tomography revealed that the NiAl precipitates were insignificantly influenced by the intercritical annealing. The strength level after intercritical annealing increased compared to an overaged condition but was lower in comparison to a standard heat treatment. Ductility behaved in an opposite way. The impact toughness was hardly improved, however the fracture toughness increased significantly by the introduction of intercritical annealing. A refinement of the grain size was not observed, which indicates the presence of the so-called austenite memory effect.