The present PhD thesis concentrates on a 11 wt.% Ni, 13.5 wt.% Co, 2.9 wt.% Cr, 1.2 wt.% Mo, 0.22wt.% C steel, which was analyzed regarding the microstructural development during heat treatment and regarding its structure property relationship. The investigated steel grade belongs to the group of secondary hardening high Co-Ni steels, which exhibit excellent toughness properties combined with high hardness and strength levels. The high toughness of these steel grades is attributed to the thin layers of reverted austenite, which form upon tempering on martensite lath boundaries and exhibit a high stability against martensitic transformations. The aim of this work was to analyze the influence of heat and cryogenic treatment on microstructural features and in further consequence also to determine the effects of microstructure on toughness properties. The microstructure of investigated high Co–Ni steel is rather complex and consists of martensite, secondary hardening carbides and retained and reverted austenite. Microstructural characterization was conducted on austenitized, cryogenically treated and tempered samples by means of atom probe tomography, scanning and transmission electron microscopy. Additionally, also the chemical composition of the present phases was determined. X-ray diffraction measurements were applied to analyze the development of austenite phase fractions due to tempering. Also, the changes of lattice constants of austenite and martensite during tempering were analyzed and related to chemical alterations. In order to evaluate formation and transformation processes of carbides, reverted and retained austenite during tempering and subsequent cooling, dilatometry investigations were carried out. Moreover, the experimental findings were corroborated with kinetic calculations with DICTRA and MatCalc to get a clear understanding of the processes during tempering and of the influence of the preceding cryogenic treatment. For correlating the results from microstructural investigations with mechanical properties, tensile tests and fracture toughness measurements were carried out. With these investigations, it was found that reverted austenite is highly enriched in nickel and thus stabilized. Conversely, the chemical composition of retained austenite does not significantly change during tempering, except for the carbon content. As the retained austenite is not stabilized, it transforms upon cooling after tempering and deteriorates the toughness of the alloy. Furthermore, it was evaluated that also the carbide precipitation processes, which are influenced by the amount of retained austenite, affect the toughness and strength properties of the investigated steel.