Progressive changes in the relationship of aircraft navigation and Air Traffic Control have produced a disturbing discrepancy between terrain clearance theory and practice. In theory, terrain clearance is assured by the aircraft adhering to certain specific and well-known traditional techniques; but in practice these are now often allowed to go by default, due to pressure of the A.T.C. environment. The purpose of this paper is to illuminate the exact nature of the problem and to suggest that clarification is overdue.