Planning in this century has been largely influenced by the appeal of scientific activity on the one hand and by reform movements on the other. As a result of this influence, two roles of planning developed with one predominating: an advisory role which adheres to the scientific canon of deta ched objectivity and which is the prevailing version, and an active role where intervention into the development of society is promoted. This duality creates problems for planning theory and practice, especially since the positivist foundation of planning has proven to be inadequate. An examination of logical positivism indicates that a correct social theory can only be critical. This suggests that planning should use critical theory as new epistemological framework.