This paper argues that the role of Computer Aided Design (CAD) should be to enhance and extend paper-based working, rather than to replace it. Current CAD tools are little used in the early stages of designing. Paper is heavily used, with its physical properties supporting work practices that are not possible with CAD. This paper presents implications for the design of new computer-based tools drawn from observations of designers' work practices. A new system is described that integrates paper-based and computer-based working in the same physical space and supports the application of computation to images on paper.