A study is made of the fracture properties of a range of three Li2O-SiO2 glass ceramics with different grain sizes. Both equilibrium and kinetic crack propagation parameters are evaluated using the controlled flaw indentation technique with a view to making reliability predictions. Fracture toughness in the well-behaved large-contact-flaw range is found to increase with increasing grain size. At low contact loads the strength and apparent toughness are found to fall below the predictions of the well-behaved region, this tendency increasing with increasing grain size. At the fixed contact load chosen for fatigue testing, the susceptibilities to slow crack growth for the three materials are found to be similar. A theory is developed to allow the variations in apparent toughness to be taken into account for lifetime prediction. The results suggest that caution must be exercised when reliability predictions are made for materials showing the effects of crack-microstructure interaction, especially in extrapolations to low contact loads. Analysis of glass ceramics can aid in their conservation.