Much progress has been made in our understanding of the Cretaceous/Tertiary (K/T) and other extinction events throughout Phanerozoic time. Scenarios for the K/T extinctions have been outlined. Suggested exogenic causes include a large asteroid impact or a number of smaller impacts over a year or soon both continental and oceanic terranes, or a comet shower of six to ten impacts over a 1- to 3-m.y. period. Suggested endogenic causes include the environmental effects of a major sea level regression and intense global volcanism.