Heterojunction band lineups have been investigated for over 40 years, and still are one of the most difficult open problems in condensed matter research-with an immediate impact on heterojunction devices. Steady progress has been made at least in understanding the ingredients of the band lineup process since the advent of surface-sensitive techniques in this field. Empirical links with the problem of Schottky barriers were discovered, and for both cases ''extreme' models based on only one ingredient had to be abandoned except in very special cases. A whole array of new and advanced instruments such as ultrabright synchrotron sources, photoemission spectromicroscopes, and free electron lasers is now being developed that will allow a rejuvenated attack to this problem, raising questions about old experiments, and opening new perspective in the very definition of interface barriers and band lineups.