In the 1990s, one of the dominant modes of computation will be in graphics, picture and image processing. All applied fields will be affected although none more so than those areas such as spatial planning and decision-making whose modes of analysis and communication are based on maps. Already, Geographic Information Systems (GIS) are becoming widespread in management and planning, and their focus and form is beginning to affect the organization and operation of policy-making. In this paper, we address the problems and potential of such systems, particularly in relation to the analytical, predictive and prescriptive basis on which such planning processes are founded. Current GIS are not rooted in the sorts of function and activities which drive the planning process and here we will identify the difficulties and possibilities for developing more appropriate GIS which are sensitive to the sorts of simulation, optimization and design activities on which spatial planning is based. To this end, we will describe the development of Planning Support Systems (PSS) in which a wide array of data, information, and knowledge might be structured, and within which GIS development must take place. We will identify the sorts of urban system and locational models which characterize planning and whose data demands might be accommodated using GIS. Our critique of GIS though is positive and constructive in that we are concerned to embed GIS into planning processes in their most relevant form. By way of conclusion, we will identify a series of requirements which PSS must meet.