The goal of this diploma thesis was to investigate possible sources for an increase in toughness in cold work tool steels while maintaining a high hardness. The desired hardness should be as high as 60 HRC combined with a fracture toughness of about 30 MPa√m. Prior to this thesis, five different experimental alloys have been designed by means of thermodynamic calculations with ThermoCalcTM and have subsequently been produced via conventional metallurgy in small scale. Hardness-tempering experiments have been carried out to determine the tempering response and the phase fractions. Special attention was put on the evolution of retained austenite during heat treatment. The strength and toughness achieved with selected heat treatments where determined by means of tensile- and fracture-mechanical tests. The results have been summarized, discussed and compared to pre-existing alloys. Also an outlook on possible further developments is highlighted.