The precisely controlled preparation of samples is of paramount importance in both solid-state physics and materials science. In addition to its success as a device production technology, molecular-beam epitaxy (MBE) is probably the most important method in this field, since it allows the growth of crystalline layer combinations with accurate dimensional control down to the atomic level. This precision would not be possible without adequately accurate characterization techniques like reflection high-energy electron diffraction (RHEED) that provide resolution on the atomic scale while at the same time being fully compatible with the crystal growth process.