Today's computer assisted design (CAD) systems automate traditional ways of working with tracing paper and pencil, but they cannot represent the rules and relationships of a design. As hardware becomes faster and memory less expensive, more sophisticated fundamental software technologies will be adopted. This shift in the basis of CAD will provide powerful capabilities and offer new ways to think about designing. Recently parametric design, a technique for describing a large class of designs with a small description in code, has become a focus of attention in architectural computing. In parametric CAD systems, design features are identified and keyed to a number of input variables. Changes in the input values result in variations of the basic design. Based on conventional software technologies, parametric design has been successfully applied in many design domains including architecture and is supported by several commercial CAD packages. A weakness of parametric techniques is the need to predetermine which properties are input parameters to be varied and which are to be derived. Relational modelling is a simple and powerful extension of parametric design that overcomes this weakness. By viewing relations as reversible rather than one-way, any set of properties can be chosen as input parameters. For example, a relational model that calculates the shadow length of a given building can also be used to calculate the building height given a desired shadow length. In exercising a relational model the designer is not limited to a pre-selected set of input variables but can explore and experiment freely with changes in all parts of the model. Co is a relational modelling environment under development on the Macintosh-II computer, and Co-Draw, a prototype CAD program based on Co. Co's relationaI engine and object-oriented database provide a powerful basis for modelling design relations. Co-Draw's interactive graphics offer a flexible medium for design exploration. Co provides tools for viewing and editing design models in various representations, including spreadsheet cards, tree and graph structures, as well as plan and elevation graphics. Co's concepts and architecture are described and the implications for design education are discussed.