Earlier publications have shown sharp peaks in the abundances of the plantinum-group metals, represented by iridium, at the Cretaceous-Tertiary (C-T) boundary in marine sediments. This led to a widely accepted theory that collision of an asteroid caused world-wide extinction of approximately half of the flora and fauna then alive. Information regarding iridium-anomalies in non-marine sediments of that period now has shown that the iridium was not extracted from sea water; sediment starvation and a nearby supernova have also been eliminated as possible sources. Gold/iridium and platinum/iridium ratios from the C-T boundary clays indicate a type C1 carbonaceous chondritic composition for the impacting object. The impact site has not been located; there is a 20% probability that the impact occurred on sea floor that has subsequently been subducted; and circular sea-floor features west of Portugal, and the Deccan Traps of India have been considered as candidates. The killing mechanism has not been established, but temperature changes and darkness due to atmospheric dust both are probable contributors. Darkness probably lasted a few months rather than a few years, as originally suggested, and arguments to support this are presented. -- AATA