The use of surface-sensitive experimental techniques has produced in recent years important advances in the understanding of the microscopic nature of heterojunction parameters. In particular, valence band discontinuity has been measured for a large number of interfaces by photoemission spectroscopy and the results identify the LCAO approach by Harrison (1977) as the best first-order calculation of this parameter. Local effects, however, limit the accuracy of the estimate to 0.2-0.3 eV. Some evidence has been obtained to show that surface defects play an important role in the interface Fermi-level pinning for heterojunctions involving III-V compounds, and therefore in the establishment of the built-in potential