At any semiconductor heterojunction there is an interface dipole associated with quantum-mechanical tunneling, which depends on the band "lineup" between the two semiconductors. When the interface dipolar response dominates, the actual band discontinuity must be close to that unique value which would give a zero interface dipole. A simple criterion is proposed for this zero-dipole lineup, which gives excellent agreement with experimental band lineups. The close connection between heterojunction band lineups and Schottky barrier formation is emphasized. © 1984 The American Physical Society.