Selective area epitaxy employing a movable shadow mask was applied to form site-controlled III-V semiconductor nanostructures on GaAs (100) in Volmer - Weber fashion by molecular beam epitaxy. In the selective area epitaxy procedure, gallium is deposited locally through apertures in the shadow mask. By making the size of the aperture comparable to the mean distance characteristic for a self-assembled nanodroplet formation, it is possible to position of a single nanodroplet with high accuracy. This requires two major steps, a shadow mask fabrication and an optimization of the droplet epitaxy method for a shadow masked selective area epitaxy. For that, the shadow mask design is developed. This is realized on silicon/silicon nitride-basis in terms of silicon process technology. Its compatibility with molecular beam epitaxy procedure is proven. In addition, a gentle in-situ cleaning of the mask treatment is found. Consequently, the droplet epitaxy method is adapted for a shadow masked selective area epitaxy. Due to the droplet epitaxy itself is a multi-step growth procedure, every single step is improved and analyzed separately using both scanning electron and atomic force microscopy. The optimization of the gallium deposition parameters is performed. A substrate temperature, a gallium deposition rate, as well as a gallium amount, were selected to obtain a single nanodroplet per aperture occupancy.