The paper describes the failure of Aznalcóllar dam, in southern Spain, in April 1998. The rockfill dam slid forward and released a flow of acid-saturated tailings. The geology and geomorphology of the site are described. The results of a detailed laboratory testing programme on the tailings material and on the overconsolidated high-plasticity foundation clay are given. Special attention is paid to the identification of cementation of the tailings and to the strength and consolidation properties of the foundation clay. Failure features are interpreted on the basis of the field evidence, taking into account the measured geotechnical properties of the materials involved.