The aim of this master¿s thesis is, to investigate the influence of deformation in the austenitic phase field on the microstructure of the maraging steel PH 13-8 Mo. The thermomechanical treatment of this precipitation hardened steel consists of solution annealing, deformation in the austenitic phase field (ausforming) and aging. In order to understand the influences of the different ausforming parameters, different states of the material were produced using a deformation dilatometer. Ausforming was performed at deformation temperatures of 400°C and 600°C and with degrees of deformation of Phi=0, Phi=0,4 and Phi=0,7. The aging temperature was varied between 482°C ¿ 593°C. Based on hardness measurements according to Vickers, it was possible to determine the hardness of the different material states. With an optical microscope the former austenite grain size as well as the structural deformation were obtained. By electron backscatter diffraction the block size of the martensite as well as the appearance of texture was observed. With the help of the data from the deformation dilatometer, it was possible, to define the martensite start temperature. Additionally, X-ray diffraction was used, to determine the different phase fractions. Subsequently, the size and composition of the precipitates were examined with high resolution atom probe tomography. Within the scope of this master¿s thesis, it could be shown that ausforming led to a significant reduction in the austenite grain size. After the martensitic transformation a noticeable refinement of the martensite structure was proven. This refinement accompanied with the rise of the hardness and the formation of deformation texture. After subsequent aging, it was found that for all aging temperatures the hardness of the deformed samples is higher than that of the undeformed reference samples. After all, the results show a strong influence of ausforming on the intermetallic precipitates as well as on the amount of reverted austenite.