This paper describes the present position in the development of a CAD package for coupled-cavity travelling wave tubes (TWTs). The objective of the work is to produce a model which has sufficient accuracy to make possible a significant reduction in the very high development costs of these tubes. If this goal is attained it will also be possible to optimise tube designs to a much greater extend than at present. The work is concentrated on a linear (small-signal) model but it is intended that there will ultimately be a nonlinear (large-signal) model also based on assumptions whose validity has been tested in the linear model. The modelling of the device is therefore concerned with the description of the electron beam, of the slow wave structure and of the interaction between them.