In this paper we describe a completely new technique that has been called low angle incidence microchannel epitaxy (LAIMCE). By LAIMCE, selective area growth and enhancement of lateral growth on the mask can be achieved in molecular beam epitaxy (MBE). In LAIMCE, the molecular beams are sent with a small angle with the substrate of about 10-30 degrees. By using LAIMCE we have successfully grown GaAs selective epilayers on GaAs (0 0 1) substrates patterned with a SiO2 mask. These epilayers have been grown laterally on the mask material with a good ratio of width to thickness such as 8 in the present experiments. Moreover, we discuss the differences between conventional MBE and LAIMCE for what concerns the degree of selectivity and the growth mechanisms. It was found that a good degree of selectivity can be obtained at growth rates as high as 1.0 mu m/h and at substrate temperatures as low as 610 degrees C. In LAIMCE, inter-surface diffusion of adatoms plays a fundamental role in determining the shape of the growing epilayers. The morphology of the side surface and the lateral growth width are influenced by the crystallographic orientation of the open windows, the angles between the beams and the substrate and by the growth conditions.