The toughening behaviour of a number of SiC fibre reinforced glass-ceramic systems was evaluated in terms of the crack growth resistance curve by performing three point bend tests on notched samples. During testing the initiation and growth of delamination cracks were observed. Afterwards a model was developed that describes the crack growth resistance of the materials. The basic assumption of the model is that the cracks are of mixed mode. They initiate from the notch in mode II, but as the crack proceeds the mode I-component becomes more important. At large crack lengths, the cracks consist almost entirely of a mode I-component. A comparison of theory and experiment shows that the model offers a reasonable description of the crack growth resistance of the composites.