The mechanical response of micron and sub-micron sized, single crystalline metallic samples strongly depends on their size. A fact which is neither expected, nor fully understood, but also a fact which has been proofed by several studies during the last decade. The reasons for this size dependency are manifold: On the one side there is a clear transition from a collectively controlled to a stochastic dislocation behavior. On the other side the size of a dislocation is naturally restricted by the sample size, which requires higher stresses for dislocation multiplication. For a thorough understanding of plasticity in small dimensions it is inevitable to observe dislocations in situ during plastic deformation, i.e. during multiplication, slip and annihilation. Besides Transmission Electron Microscopy (TEM), which enables the observation of single dislocations, synchrotron based micro-diraction techniques (as for instance µLaue-diraction) can provide insights into the collective behavior of dislocations. Aim of this thesis is to contribute to the understanding of size dependent plasticity by µLaue diraction experiments. The thesis is devided in three parts: (i) Post mortem µLaue diraction on samples which have been deformed in situ in the Scanning Electron Microscope (SEM); (ii) Design of a straining device able to perform in situ µLaue experiments on micron sized samples under tension; (iii) In situ compression and tensile experiments on micron sized single slip oriented Copper samples during µLaue diraction. The results show (i) the unpredicted activation of a slip system at low strains and allow for the estimation of schematic slip mechanism maps; (ii) A device which is able to perform tension, compression and bending, as well as bending fatigue, for samples at the micron scale with an accuracy of approximately 10µN and 1nm; (iii) Tensile experiments on approximately 6µm sized samples which show expected and unexpected plastic behavior at the micron scale. Furthermore, the impact of experimental constraints and imperfections on the dislocation structure is shown by in situ compression tests.