In his article author discusses some methodological problems which confront every researcher in the field of man power planning. Especially the use of the need concept offers difficulties. The belief that the need of physicians could be assessed objectively is a false one: it is based on the desire to develop a health system with the greatest possible (internal) efficiency and (external) effectiveness. The means at our disposal however, are limited. How to distribute them over the different sectors of society is a political question. The same problems arise when we have to decide how to distribute them over the different parts of our health system. This has consequences for prognostic thinking and for planning activities in the medical sector.