For people with various forms of physical disability, extreme slowness of communication is commonplace. In the first part of this 1992 book a functional architecture for communication aids is discussed and the idea of automatically supplying the intrinsic redundancy contained in natural communication is explained. The distinctions between adaptive and non-adaptive models of communication are shown and details are given of working predictive text generation systems. One such system is the Reactive Keyboard, and in the second part of the book this is described. It greatly speeds communication by predicting the user's next response, although it does not always predict correctly. The guesses are made on the basis of previous answers and thus can conform to whatever kind of text is entered. This will be of great value to all involved in helping disabled users interact with computers.