Basic methods used in conventional and high resolution electron microscopy are briefly reviewed including the parameters limiting resolution. In addition, advantages of convergent beam electron diffraction are discussed. Two specific examples (the role of stoichiometry in GaAs layers grown by molecular beam epitaxy at low temperature and defects formed at InGaAs/GaAs heterointerfaces) illustrating applications of electron microscopy to the understanding properties of semiconductor heterolayers are described. Electron microscopy is revealed as a very powerful tool in solving problems in material science. It is extremely useful for achievement of desired optical and electrical characteristics.