Snakes are a diverse group of long‐bodied, limb‐reduced reptiles closely related to lizards; together they are termed squamate reptiles. Fragmentary snake fossils first appear in the Jurassic Period (∼167 million years ago (Ma)), with more complete fossil snakes, many with vestigial legs, appearing during the Cretaceous Period (∼90 Ma). Snakes have since diversified rapidly, and now comprise ∼3800 species that occupy terrestrial, subterranean, arboreal and aquatic niches. All snakes are predators, having highly mobile skulls and jaws that can often swallow prey much larger than their own diameter. Primitive snakes such as boas, pythons and blindsnakes lack venom fangs. Such fangs have evolved repeatedly within more advanced snakes, called the Colubroidea, which make up ∼80% of the diversity of living snakes: mobile front fangs in viperids (e.g. vipers and rattlesnakes) and atractaspidids (stiletto snakes), fixed front fangs in elapids (e.g. mambas and cobras), and rear fangs in many lineages of colubrids (e.g. boomslangs and twig snakes). Key Concepts Snakes are just one of the many lineages of limb‐reduced lizards – though by far the most successful lineage. Snakes have highly flexible (kinetic) skulls and lower jaws, and many forms can engulf prey much wider than their heads. Snakes are one of the most rapidly diversifying groups, with around 3800 living species, mostly belonging to the Colubroidea. Venom‐delivery systems (fangs and glands) have evolved multiple times within the most advanced snake lineage (Colubroidea). Primitive fossil snakes such as Najash, Eupodophis, Haasiophis, Pachyrhachis (∼90 Ma) retain prominent external hindlimbs, as would be expected given that snakes evolved from lizard‐like reptiles.