The Scolecophidia, commonly known as blind snakes or thread snakes, are an infraorder of snakes. They range in length from 10 to 100 centimetres. All are fossorial. Five families and 39 genera are recognized. The Scolecophidia infraorder is most likely paraphyletic.

The Alethinophidia are an infraorder of snakes that includes all snakes other than blind snakes and thread snakes. Snakes have long been grouped into families within Alethinophidia based on their morphology, especially that of their teeth. More modern phylogenetic hypotheses using genetic data support the recognition of 19 extant families, although the taxonomy of alethinophidian snakes has long been debated, and ultimately the decision whether to assign a particular clade to a particular Linnaean rank is arbitrary.