Episodes of mass extinction evident in the fossil record of ancient life have evoked many hypotheses by way of explanation. One of the most celebrated of these episodes, the Cretaceous-Tertiary boundary, occurred about 65 million years ago when the dinosaurs and many other animals disappeared. A very appealing mechanism, for which ample evidence has been revealed, was the asteroid impact hypothesis advanced twenty years ago. Considerable subsequent research has been devoted to relating other mass extinctions to the same mechanism, but with only ambiguous success. On the other hand, modern radioisotopic dating has revealed that huge volcanic events correlate very well with mass extinctions, including that at the Cretaceous-Tertiary boundary, and many now suspect that atmospheric effects of volcanogenic gases are culpable. A possibly broader role for impacts, perhaps even as causative agents for volcanism, is currently unresolved.