The Mesozoic Era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the Age of Reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time. The era began in the wake of the Permian-Triassic event, the largest mass extinction in Earth's history, and ended with the Cretaceous–Paleogene extinction event, another mass extinction which is known for having killed off non-avian dinosaurs, as well as other plant and animal species.

Mesozoic means "middle life", deriving from the Greek prefix meso-/μεσο- for "between" and zōon/ζωον meaning "animal" or "living being". It is one of three geologic eras of the Phanerozoic Eon, preceded by the Paleozoic ("ancient life") and succeeded by the Cenozoic ("new life"). The era is subdivided into three major periods: the Triassic, Jurassic, and Cretaceous, which are further subdivided into a number of epochs and stages.

The Mesozoic was a time of significant tectonic, climate and evolutionary activity. The era witnessed the gradual rifting of the supercontinent Pangaea into separate landmasses that would eventually move into their current positions. The climate of the Mesozoic was varied, alternating between warming and cooling periods. Overall, however, the Earth was hotter than it is today. Non-avian dinosaurs appeared in the Mid-Triassic and became the dominant terrestrial vertebrates early in the Jurassic, occupying this position for about 135 million years until their demise at the end of the Cretaceous. Birds first appeared in the Jurassic, having evolved from a branch of theropod dinosaurs. The first mammals also appeared during the Mesozoic, but would remain small and modest until the Cenozoic.