Materials consisting of grains or crystallites with sizes below a hundred nanometers have exhibited unique physical and mechanical properties in comparison to their coarse-grained counterparts. As a result, considerable effort has been put into uncovering the new deformation mechanisms that give rise to this outstanding response of nanocrystalline materials. Moreover, the production of nanocrystalline materials of reasonable sizes for structural applications remains a challenge. However, the size limitation is of no issue for their present application in the growing field of MEMS and NEMS devices. Ultimately, the reliability and lifetime prediction of these devices will depend on the accurate knowledge of their mechanical response. This dissertation addresses experimental and simulation procedures used to understand the fundamental deformation mechanisms operating in bulk nanocrystalline Nickel. Recent results from simulations suggested dislocations as a dominant carrier of plasticity in nanocrystalline materials. In contrast to coarse-grained materials, these dislocations are nucleated at grain boundaries and, after propagating through the nano grains, they are absorbed there as well. Deformation experiments during in-situ X-ray diffraction strengthened the predicted outcome from simulation but many open questions remained. Within this thesis a more extensive range of in-situ testing experiments are performed that aim to systematically investigate the nanocrystalline deformation mechanism in terms of both temperature and external loading conditions. The development of a low temperature tensile test set-up allowed to study temperature dependent behavior and revealed that dislocation activity in nanocrystalline Nickel is a strongly thermally activated process where propagation of dislocation seems to be as important as nucleation from dislocations at the grain boundary. This finding is further supported by strain-dip tests, which revealed that pinning points strongly influence dislocation propagation. Nanocrystalline Nickel exhibits, in its as prepared state, large internal stress variations. These stress variations and the small grain size are most likely responsible for the microplastic regime characterized by an extended macroscopic strain, making the usage of the classical definition of yield questionable. Furthermore it could be shown that upon annealing, which reduces the samples' internal stress, this extended microplastic regime was observed to be less pronounced. To study the structural stability of nanocrystalline Nickel at large strains, the material was investigated by compression experiments, revealing no changes in mean grain size. Furthermore, the three dimensional atomic probe technique was utilized for localizing impurity concentrations. The program was rounded by calculating diffraction peaks from simulated nanocrystalline structures with a single type of defect. This allowed investigating the characteristics of the diffraction pattern of nanocrystalline systems in a bottom up approach. Finally, the results of the thesis are discussed in terms of a thermally activated deformation mechanism that involves the nucleation, propagation and absorption of dislocation within the nanocrystalline environment.