We investigate molecular beam epitaxy (MBE) regrowth through shadow masks developed from AlGaAs/GaAs layers on GaAs [001] substrates. Adjusting the directions of the molecular beams relative to the masks results in in situ lateral structuring. This enables us to modify doping and composition within one layer, to shift it laterally, and even to split it or to reduce its width relative to the mask's aperture. The resolution of this selective area epitaxy (SAE) technique is restricted by half shadows and surface diffusion. With diaphragms and thin masks it is possible to decrease half shadows to a few nanometers. However, diffusion mainly is a material parameter. We have investigated III-V and II-VI semiconductors. In the latter case it is extremely dependent on the crystal orientation. Along [1-10] diffusion lengths are very short. This is in agreement with ab initio calculations in our group. Thus lateral structuring with a resolution of a few nanometers is achieved. We discuss the degrees of freedom, which arise from different chamber geometries and mask concepts. With simple striped masks complex structures such as stacked three colour detectors or selective contacts to embedded 2DEGs can be realised. Three novel mask concepts will be presented. Deeply underetched masks serve as elastic substrates for misfit layers over the critical thickness without plastic relaxation. Second, as an analogy to cleaved edge overgrowth, we use masks developed from AlGaAs/GaAs superlattices. Finally interconnected structures with lateral periodicity of a few micrometers have been realised with grid masks.