Tool steels cover a variety of carbon and alloyed steels which are suitable for being used as tools. One sub group of the tool steel family is the hot-work tool steel group. Typical applications are the use as die casting equipment mainly for the light alloy processing, forming dies, die inserts or plastic moulding dies. Chromium hot-work tool steels are most frequently used for high-temperature applications because of their high toughness and shock resistance. Plastic mould steels are very similar providing higher chromium contents, hence, increased corrosion resistance, which is required for the processing of chemically aggressive plastics. The typical heat treatment of tool steels consists of an austenitisation treatment with a subsequent quench, followed by a multiple tempering procedure. After quenching, the microstructure consists predominantly of a super-saturated martensitic matrix with amounts of retained austenite and some embedded primary carbides which have not been dissolved during austenitisation. Tempering leads to a relaxation of the martensitic matrix, which increases toughness, and to the formation of secondary carbides. The latter are responsible for the outstanding high-temperature strength and the tempering-resistance of these steels. The production of large components requires corresponding moulds and dies of large dimensions. From that, cooling rates during hardening are limited in the inner regions. Tool steel producers observe reduced toughness values at lowered cooling rates during hardening which can cause catastrophic failure during operation. This phenomenon also occurs when the microstructure is predominantly martensitic, hence, no pearlite or bainite is present. Thus, the aim of the present work is the identification of the microstructural origin of this cooling rate dependent material behaviour within the “purely” martensitic range. For this, two different tool steels showing similar cooling rate dependent toughness behaviour have been investigated. First, chromium hot-work tool steel X38CrMoV5-1, and second, plastic mould steel X38CrMo16. The combination of dilatometry, impact bending testing and high resolution characterisation methods as scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atom probe tomography (APT) lead to the clarification of the described phenomenon for both materials. Fernlike/dendritic pro-eutectoid cementite at former austenite grain boundaries which forms directly from the austenite during hardening has been found to be responsible for the toughness reduction at low cooling rates in case of the plastic mould steel. In case of the hot-work tool steel, carbon enriched interlath retained austenite films which form laminary arranged carbides during tempering have been found to influence toughness. The thickness increase of these films with decreasing cooling rates leads to an increased potential for laminary arranged carbides at low cooling rates which enhance crack propagation. The gained results, especially in case of the hot-work tool steel give reason to critically reassess the commercial heat treatment process.