The character of interdisciplinarity in planning approaches create a new, intriguing, emerging complexity (Funtowitcz and Ravetz, 1994) in problems and in knowledge-structuring of contexts of planning practices. The key-role played by information systems (IS) implicates a re-consideration ofcharacter of knowledge to be used in knowledge-bases. The necessity of considering knowledge domains coming from social, cultural, economical, technical, physical and naturalistic approaches means dealing with different scales of value, with non homogenous parameters. The necessity ofmanaging flexible knowledge rises on the fore as fundamental issue for future information system oriented to supporting decisions. Might information systems be useful in this interdisciplinary approach ? It is necessary to contain in a knowledge-base both quantitative and qualitative information. Three alternatives are available for a conceptual discussion:the possibility of identify new approaches, in order to develop information systems able in managing new knowledge, the necessity of adding new support systems oriented to manage soft knowledge, to traditionalgeographic information systems (GIS), the possibility of non using support systems coming from a technological vision of problem for nontechnical knowledge (Latouche 1996). The first two paragraphs are due to F. Selicato. The third and the fourth paragraph are due to C. Torre.