An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp fall in the diversity and abundance of multicellular organisms. It occurs when the rate of extinction increases with respect to the background extinction rate and the rate of speciation.

Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty. These differences stem from disagreement as to what constitutes a "major" extinction event, and the data chosen to measure past diversity.

In a landmark paper published in 1982, Jack Sepkoski and David M. Raup identified five particular geological intervals with excessive diversity loss. They were originally identified as outliers on a general trend of decreasing extinction rates during the Phanerozoic, but as more stringent statistical tests have been applied to the accumulating data, it has been established that in the current Phanerozoic Eon, multicellular animal life has experienced at least five major and many minor mass extinctions. The "Big Five" cannot be so clearly defined, but rather appear to represent the largest (or some of the largest) of a relatively smooth continuum of extinction events.

The "Big Five" of the Phanerozoic Eon were anciently preceded by the presumed far more extensive mass extinction of microbial life during the Great Oxidation Event (also known as the Oxygen Catastrophe) early in the Proterozoic Eon. At the end of the Ediacaran and just before the Cambrian explosion, yet another Proterozoic extinction event (of unknown magnitude) is speculated to have ushered in the Phanerozoic. Several events in the Cambrian and early Ordovician meet or exceed the "Big Five" in proportional severity, though overall diversity was rather low until the Great Ordovician Biodiversification Event (GOBE). Sepkoski and Raup (1982) initially tracked absolute (rather than proportional) extinction, so their biodiversity estimates overlooked events prior to the GOBE.

End Ordovician or O–S, just prior to and at the Ordovician–Silurian transition. Two events occurred that killed off 27% of all families, 57% of all genera and 85% of all species. Together they are ranked by many scientists as the second-largest of the five major extinctions in Earth's history in terms of percentage of genera that became extinct.

In May 2020, studies suggested that the causes of the mass extinction were global warming, related to volcanism, and anoxia, and not, as considered earlier, cooling and glaciation. However, this is at odds with numerous previous studies, which have indicated global cooling as the primary driver. Most recently, the deposition of volcanic ash has been suggested to be the trigger for reductions in atmospheric carbon dioxide leading to the glaciation and anoxia observed in the geological record.

The Late Devonian extinctions were a series of events that occupied much of the Late Devonian up to the Devonian–Carboniferous transition. The Late Devonian was an interval of high diversity loss, concentrated into two extinction events. Scientists have linked both events to anoxic conditions in the water.

The larger extinction was the Kellwasser Event (Frasnian-Famennian, or F-F, 372 Ma), an extinction event at the end of the Frasnian, about midway through the Late Devonian. This extinction annihilated coral reefs and numerous tropical benthic (seabed-living) animals such as jawless fish, brachiopods, and trilobites. Many scientists believe that the Kellwasser event resulted from land nutrients being carried into the ocean by rivers. These nutrients caused massive algal blooms. As the algae died and decomposed, they consumed dissolved oxygen in the water column, leading to anoxic conditions which eventually caused the extinctions.