It is generally felt that by causing aircraft to follow accurate climb and descent paths the flow of traffic in terminal areas can be improved. This paper, which was presented at a meeting of the Institute held in London on 29 November 1972, with the President, G. E. Beck, in the Chair, discusses the special constraints involved in vertical plane navigation. These arise from difficulties in the accurate measurement of position, and the limitations of aircraft response. The need to fly at differing speeds and in various configurations, while minimizing passenger discomfort and external noise, is also considered.