The Red List of Threatened Animals is a warning flag, signaling for global attention to the perilous status of many species. In its approach, this Red List is different from all its predecessors and the existing Red Data books. It employs a more rigorous system for assessing degree of threat of extinction, a system reflecting current knowledge in population biology in relation to conservation concerns. This more objective approach should help focus attention on the causative factors behind the diminished geographic ranges, declining populations, and small numbers that characterize endangerment in many species.