The mechanical properties of tool steels are specially influenced by the amount, size and type of different microstructural features. The heat treatment is one of the last steps in the production process of tool steels. It influences the prevailing microstructural features and, therefore, the properties to a high degree. The optimal and desired function of tool steel can be set up by the heat treatment. Thereby, many metal-physical principles are used such as diffusion, recrystallization, grain growth, precipitation behaviour, solution behaviour of the alloying elements and lattice transformations. Recently, a cryogenic treatment, which is considered to be an integrated part of the heat treatment, is becoming more important because of its positive effect on the lifetime of tool steels. In this work the effects of deep cryogenic treatment on the static and dynamic mechanical properties of tool steels were investigated. Different mechanical tests such as tensile tests, impact tests, tests for fracture toughness and fatigue were carried out. Additionally, SEM-analysis, XRD-measurements and optical-microscope examinations were performed.