This paper has considered the issue of clay in rocks and its effect on the performance of aggregate and bituminous mixtures. It has been shown that of the many types of clay present, not all are detrimental to the performance of asphalt mixtures. Some are inert and can be considered simply as filler, whereas others are moisture sensitive and need careful assessment. This assessment is not a simple process. Consideration has to be given to in-service performance i.e. is loss in aggregate strength the issue, or can the presence of expansive clays account for unexpected mixture properties such as increase in retained stiffness?