By utilizing special manufacturing conditions, e.g., using only pure elements and applying a rapid cooling rate, tool materials with high quasi-static fracture toughness can be produced. However, tool materials are often subjected to cyclic loading and, hence, their lifetime is dominated by fatigue failure. This study is focused on fracture mechanics and in-situ experiments to characterize the fatigue crack propagation behavior of three newly developed tool steels at a stress ratio R of 0.05. Microstructural examinations revealed that the materials consist of the phases alpha'-martensite, retained austenite, and complex carbides in different amounts. Results of preliminary tests are presented, in which it was attempted to grow the crack in a plane parallel to the plane of the starter notch. The determined a dagger K threshold values ranged between 4 and 5 MPaaem with Paris-Erdogan exponents of 3.3-4.6. In-situ observations were performed to understand the inherent damage mechanisms and microstructural effects during fatigue loading. These observations showed that fatigue crack growth is mainly dominated by the ductility of the martensitic-austenitic matrix. Only in cases in which the primary carbides are oriented favorably (with respect to the direction of crack propagation) does the crack follow the coherent carbide network to a certain extent. Furthermore, for the first time, a phase transformation from retained austenite to alpha'-martensite was detected at the crack tip during fatigue crack propagation for the material group of tool steels.