The purpose of this paper is to discuss en-route separation criteria applicable to the supersonic transport. Separation standards are described; equations are given and analysed which show the factors influencing separation; finally, suggestions are made for a supersonic transport air traffic control system for the North Atlantic. The discussion is concerned with longitudinal and lateral en-route separation in a non-radar environment, as opposed to radar separation in the terminal areas. The terminal problems are so complex as to require separate consideration and have been investigated by other authors. An appendix on the derivation of equations expressing statistical parameters of errors and estimated aircraft separation has here been omitted.