The cause of the Cretaceous/Tertiary extinction has become one of the major geologic controversies of our day. Many scientists now support the theory that the massive extinctions were caused by an impact on the earth by an extraterrestrial object. Other scientists believe that the anomalously high concentrations of Iridium that are used to support meteoritic or cometary impact have a terrestrial source, either gradual or catastrophic. The current evidence is reviewed with the intention of providing an introduction to the controversy and to form the basis for a seminar or disscussion topic.