Planning theory has, as may be seen, been heavily influenced by theoretical developments in other academic disciplines, notably economics, politics and sociology. The reason for pointing to this is not to suggest that this is in some way undesirable, for it is normal that ideas developed in one sphere of activity should migrate to other areas (Mulkay, 1979). Rather, it is important to recognize the common theoretical origins which often underlie what can appear to be opposed frameworks and to understand the nature of the interests served by their appearance upon the scene. The planning theory which first began to be widely discussed in planning education in the 1960s was almost exclusively American in origin. Although it contained many different facets which, as in the case of the disagreements between rational-comprehensive planning theorists and incrementalists or the later rejection of the former by adherents of advocacy planning it shared certain key assumptions (Simon, 1955; Lindblom, 1973; Meyerson and Banfield, 1955; Davidoff, 1973).