In an earlier paper I suggested that ATC could be depicted as a hierarchy of processes which differ from each other in the distance ahead in time at which they attempt to solve their problems. When the look-ahead time is very long (several weeks, say) we are usually concerned with the problems of traffic en-masse. These problems are here regarded as the province of Air Traffic Management (ATM) rather than of ATC, which is concerned in general with issuing commands to individual aircraft. It is possible that ATC may attempt to plan say, one hour ahead, but there will be a residue of problems, some of which may require action within seconds. The relatively long-term (or upstream) process is often called ‘strategic’ ATC and the short-term (or downstream) process ‘tactical’ ATC. The present paper is concerned with the problem of distributing the control effort between upstream and downstream activity. Other things being equal, upstream planning can offer a more efficient flight and a more orderly process, with less immediate pressures on pilot and controller. One drawback is that, in Western Europe at least where there are a multiplicity of agencies, the ATC authority may not have adequate data to attempt to plan more than about 15 minutes ahead.