In a procedural Air Traffic Control environment, navigation standards are related to separation standards because of the way they both affect the collision risk. The Air Traffic Studies division of the Mathematics Department at R.A.E. has developed methods of estimating the collision risk procedural systems and these provide rational grounds for decisions on separation standards. In a radar-monitored system navigation accuracy and separation standards both affect the workload of the controller on the ground and it is necessary to study these effects to ensure that he is not overloaded. In addition the problems of estimating the collision risk will become more complicated. I shall very briefly describe the mathematical theory of collision risk for parallel tracks. A fuller description has been published in the Journal.