In the present study the main objective was to investigate if it is possible to reach hardness levels over 70 HRC in high speed steels. Various approaches have been pursued with regard to a change in the hard phase content. Only powder metallurgical high speed steels were used. All the alloys have been extensively characterized. The microstructural characterization was mainly done with SEM but also with TEM, DSC and atom probe measurements. As mechanical tests hardness, fracture toughness, tension- and compression investigations were carried out. Based on the ledeburitic steel Böhler S290 MicroClean two alloys with primary carbide content up to 26 Vol% were developed. Despite the 6 Vol% higher carbide content compared to the S290 PM no considerable increase in hardness was observable. Furthermore, three model alloys with up to 26 Vol% carbonitrides were produced. Nitrides and carbonitrides form a finer and more homogeneous structure than carbides due to their higher thermodynamic stability. Nitriding was achieved by a powder nitration process. Since no reference values were available for nitriding of high-alloy tool steels, comprehensive studies were carried out on the S290 PM. The three nitrogen-modified alloys also showed no significant increase in hardness, but a very fine carbonitride structure. The second base material investigated in this work was the carbon free ternary system Fe-Co-Mo, which hardens by the formation of an intermetallic -phase. Through a powder nitration process up to 10 Vol% vanadium nitrides were induced. Mechanical tests on this modified alloys again showed no noteworthy increase in hardness. In the ensuing discussion it is stated that the total hardness is mainly determined by the matrix strength, and that only at much higher phase contents a significant hardness increase will be possible.