The design of the user interface for an augmentative and alternative communication (AAC) or computer access system is a critical factor in determining a user's p0erformance with a system. A comprehensive, quantitative, and accurate model for alternative access systems is needed to optimize both developers' design decisions and clinicians' system recommendations. This paper presents an application of one possible model, called the GOMS (Goals, Operafors, Mefhods, Selecfion Rules) model (Card, Moran, & Newell, 1983). The model provides a comprehensive description of user behavior based on system-specific parameters as well as the cognitive, perceptual, and motor capabilities of the user. It can be used to predict both task execution and learning times, as well as points of excessive long or short term memory load. The GOMS model is applied here to three interfaces currently used in AAC and computer access systems in order describe and predict user performance, both qualitatively and quantitatively. The thre...