This project involves characterization of structural defects on crystals, as well as investigations of the role of these defects in surface atomic diffusion. The mechanisms by which atoms move on surfaces to produce morphology changes and surface phase transitions will be studied. The major experimental methods for the structural and kinetic studies will be scanning tunneling microscopy, low energy electron diffraction, and surface x-ray diffraction. Experiments will be performed on semiconductors (silicon, germanium and gallium arsenide) and on metal single crystals.