Hot-work tool steels are used in engineering applications, especially as tool material for hot-pressing, extrusion and forging, offering high strength, high toughness and a good tempering resistance. This is achieved by using materials which have a high micro-cleanliness and homogeneity over the entire cross section, usually with a martensitic microstructure. However, due to the rising demand for increasing tool dimensions the cooling rate during quenching can become a major problem. For instance, depending on the alloy composition dies made by classical hot-work tool steels show an increase of bainite attributed to the temperature gradient between the centre and the surface of large components. This change in microstructure is accompanied with the formation of different precipitates, which in turn affect the mechanical properties. Consequently, the aim of this work is to gain a fundamental knowledge about how the mechanical properties are determined by the microstructure which is governed by the alloying elements and heat treatment. The studies were performed on hot-work tool steel grades X38CrMoV5-1 and X37CrMoV5-1. Based on the continuous cooling transformation diagrams determined experimentally, different hardening processes had been chosen to achieve well defined martensite, bainite or mixed microstructures. Mechanical tests such as tensile tests, fracture toughness tests and instrumented impact tests were performed in order to obtain the strength, deformation behaviour, impact toughness, and plane strain fracture toughness of the engineering material. The results revealed that the mechanical properties degrade with decreasing cooling rates. Microstructural investigations performed using light microscopy, scanning and transmission electron microscopy, three-dimensional atom probe, and small-angle neutron scattering were used to study the features relevant for the mechanical properties. These methods cover all length scales from millimetre to nanometre, which include the determination of grain size, chemical composition and crystallographic structure of the occuring precipitates, and their density and size distribution. The results showed that the degraded mechanical properties are related to the presence of the bainitic microstructure connected with M7C3 carbides which are present at former austenite grain boundaries as well as martensite and bainite lath boundaries. These investigations shall provide basic information necessary for alloy design, optimisation of heat treatments and for the reliability of hot-work tool steels during performance.