The mechanical properties of materials strongly depend on their size when sample dimensions reach below several micrometers. Methods developed to conduct compression, tension and bending experiments on miniaturized single crystalline samples made it possible to understand this mechanical size effect that leads to an increase in yield strength with decreasing sample size. However, the majority of commercially used metals or alloys are multicrystalline, i.e., are an assembly of numerous single crystalline grains. Micromechanical testing provides the perfect platform to select specific grain boundaries and produce samples that incorporate a single boundary or multiple boundaries of the same type. This allows studying the effect of one grain boundary type on the deformation behavior. The use of in situ deformation techniques in the transmission electron microscope (TEM) adds the benefit of observing dislocation-grain boundary interactions while recording force-displacement data. This thesis is aimed at further understanding the influence of grain boundaries on the mechanical behavior. Taperless Cu pillars of different sizes, reaching from 7 µm down to 200 nm in width were manufactured using focused ion beam (FIB) milling techniques and tested in situ inside a scanning electron microscope or a TEM. Bicrystalline samples incorporating an arbitrary large angle grain boundary parallel to the compression direction show an increase in yield strength and hardening and less serrated flow. Samples with a single coherent twin boundary along the compression axis on the other hand exhibit similar deformation characteristics as single crystals of the component grains. In situ TEM and scanning TEM deformation experiments on twinned samples confirm the lack of strengthening and no stress concentrations due to the twin boundary can be found in the tested orientation. A stochastic variation of dislocation density was found in the 200 nm sized samples, but never a complete dislocation starvation. Further experiments on 1 µm sized compression pillars comprised of several twin lamellas inclined to the compression direction reveal that the majority of deformation is taking place parallel to the boundaries.