Steels containing a martensitic microstructure provide a range of advantages concerning mechanical and technological properties, when compared to other steelgrades. They show highest tensile strengths for a given alloy composition and by applying a quench-and-temper-treatment an excellent combination of tensile strength and ductility can be achieved. Therefore martensitic steels are an important field of activity in materials science and engineering. In this work, a steel containing partly tempered martensite in the as-rolled condition is investigated. This material is subjected to tempering treatments in the temperature-range between 150 and 700 °C. The changes in microstructure caused by this procedure are investigated by means of light-microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), small-angle neutron scattering (SANS), X-ray diffraction (XRD), dilatometry and by magnetic and chemical analysis. The changes of mechanical properties are interpreted in terms of the results of these characterisation methods. Major correlations between the microstructure and the behaviour of the material in the tensile test, the Charpy-V notch toughness and forming behaviour are established. The results of the SANS experiments are in qualitative agreement with the results of the other investigation methods, but due to the complexity of the material they do not lead to reliable quantitative conclusions.