A model is developed for the deformation and fracture of tough fibre-reinforced ceramic matrix composites for use in component design. The model is based on a single continuum damage state parameter given by the ratio of the number of failed to unfailed fibres. The composite is assumed to be instantaneously elastic and the stress-strain response is determined from the stress-damage history. The model includes the effects of debonding and fibre pull-out, through weakest fibre Weibull statistics, and estimates of composite ductility have been made. The model quantitatively describes the dominant mechanism of deformation and fracture, and in the following paper, it is shown how the model may be used in component design.