Epitaxial InAs layers were grown by molecular beam epitaxy (MBE) on GaAs substrates. The initial stages of nucleation were studied by in situ reflection high energy electron diffraction (RHEED). Cross-sectional TEM examination was used to investigate the morphology of the growing layer, while plan-view examination revealed the generation of misfit dislocations. The growth mode was found to depend mainly on the conditions used to nucleate the epitaxial layer. In most cases, Stranski-Krastanov type of growth was observed.