This chapter reviews the data pertaining to the Cretaceous/Tertiary (K/T) and other extinction events in light of a developing model of coupled core-mantle thermal release (via periodically accelerated plume activity). In this model the D″ layer becomes unstable, causing an increased frequency of geomagnetic reversals and sending plumes of lower mantle material upward toward the lithosphere. The arrival of this material at the Earth's surface causes increased tectonism and associated climatic and sea-level changes, and thus causes accelerated preboundary extinctions. Massive flood basalt volcanism, which correlates one-to-one with every mass extinction in the Mesozoic and Cenozoic, produces progressive climatic deterioration and a carbonate crisis in the oceans. We propose that the plume model can better account for much of the evidence associated with mass extinctions. Although an endogenous mechanism for the production and distribution of shocked quartz at the K/T boundary remains problematic, it is further proposed that the iridium and other chemical signatures across the K/T boundary are equally well explained by terrestrial mechanisms.