Single crystal deformation unveiling the characteristics of crystal glide and the role of defect propagation herein has been a matter of intense research more than 60 years ago. A manifold of different mechanical testing methods and crystal qualities were investigated to gain understanding in parameters that influence the mechanical response of macroscopic single crystals. It became clear at that time how sensitive the crystal's plastic flow behavior is to both intrinsic and extrinsic parameters, where the first relates to the initial perfection of the crystal and the latter pertains to mechanical effects arising from the testing environment. From these macroscopic single crystal deformation experiments, fundamental insights were gained on micromechanistical processes and spatial dynamics of the evolving dislocation structure. But how is a crystal behaving when its geometrical dimension shrinks towards the length scale of the internal dislocation structure? This question was hard to address at the golden age of single crystal research because of the difficulty in controlling micromechanical testing and the non-availability of small samples. Today we are steering towards a "nano-society". Sample preparation and instrumentation to perform such experiments are now routinely possible, which allows investigating the posed question. This is the context, in which the present work is to be placed. This dissertation presents data obtained with a unique and specially developed in situ micro compression device, designed to operate in combination with a micro focused polychromatic x-ray beam impinging on the microscopic single crystalline cylindrical sample (e.g. a pillar) that is to be deformed. The particular choice of combining micro compression and micro focused Laue diffraction was pursued to enable non-destructive probing of the deforming microcrystal, providing a step beyond micro compression alone. At the beginning of this thesis work, the still young literature on micro compression results revealed a large sample size-effect for the strength of microcrystals – as the external dimension decreases the strength was found to increase. Rationale for the geometrical size-effect of microcrystals has been debated, since strengthening interface-effects, as encountered in polycrystals, are absent in single crystals. New dislocation mechanisms were proposed to account for the size-effect. These models were established without knowing the evolution of the microstructure within the deforming crystals. A non-destructive experimental technique was needed to address this evolution. This demand was met by the unique combination of micro compression and micro diffraction, and constitutes a pioneering effort to investigate small scale plasticity by means of Laue diffraction as a function of strain. The results of this thesis work provide an in situ view on the small scale plasticity occurring in micro crystals that is richer than that derived from mechanical testing alone. Static diffraction experiments on the as-prepared metallic micro crystals reveal a wide range of pre-existing defect structures, highlighting pre-existing internal length scales not accounted for so far. Experiments on etched and ion milled Si pillars suggest that the detected defect structures partly are a result of the ion milling sample preparation technique. In situ diffraction experiments found that single crystal metallic micro pillars are not deforming via simple laminar slip on glide planes. During the initial loading curve the diffraction peaks reveal increasing plastic strain gradients in the illuminated volume. Local plasticity is observed in the apparent elastic loading regime. In situ micro compression tests on single crystal Au, Ni and Cu micro pillars demonstrate via the dynamics of the Laue spots that an evolving dislocation structure is present in the pillars. Furthermore, it could be shown that all samples exhibit local lattice rotations, which are a signature of constrained mechanical testing conditions leading to an in-homogeneous deformation with stress gradients and the local storage of geometrically necessary dislocations. Resolving the diffraction signal in 2D over the entire pillar clearly reveals asymmetric loading conditions and the development of rotational gradients between the pillar bottom and pillar top. The 2D mapping also suggests local plasticity to be initiated at the interface between the compression tool and the sample. Altogether no evidence of new dislocation mechanisms related to the external dimensions can be supported. In contrast, the dynamics of the Laue spots evidence that internal length scales associated with the evolution of dislocation structures, suggesting strain hardening, are important. The presented data demonstrates that the flow response of micropillars is highly affected by extrinsic constraints, such as the sample geometry and the loading conditions. The sample size-effect in micropillars is therefore not derived under the absence of strain gradients, as is commonly believed.