This document presents a brief summary of the failure of Azna1cóllar dam, a catastrophic slide that caused one of the worst environmental disasters in Spain. The account inc1udes a description of the failure and of the geometry and features of the slide and gives sorne information on the analyses carried out both of initiation of the failure and of the post-failure events. It was found that the main causes underIying the failure were the occurrence of progressive failure in the brittle foundation c1ay and the presence of very high pore water pressures in the foundation. The mechanism of the failure also explains the large post- failure movements (more than 50 m) that were responsible for the large spill of tailings.