The processes involved in the formation of clay minerals are summarised. This considers the different types available, their properties and distribution within differing types of rock. The relationship between harmful types of clay and specific rock types is given. The paper then describes standard and non-standard methods that have been developed to determine the presence of clay minerals and assess their effect on aggregate and asphalt properties. The paper concludes by emphasising the need to consider the influence of clay minerals when trying to predict the performance of aggregate and asphalt materials used in highway construction.