Boas form a pan-tropical and sub-tropical family of constricting snakes. Though famous for giants such as anacondas, some boids reach a maximum length of only a little over 18 inches. 

Boids posses a pair of reduced rear limbs that are partially visible as a pair of spurs on either side of the vent. In males, spurs are longer and more conspicuous than in females.

Anatomical features distinguish boas from pythons (which have long been considered subfamilies within the Boidae)--for example, pythons lack premaxillary teeth, which boas have--as does reproductive mode. Boas give birth to living young, while pythons lay eggs which are incubated by the female. Most species have a head with a broad rear portion, giving them a triangular shape. Species that feed on birds may have very long fang-like teeth at the front of the jaw.

Most boas feed on warm-blooded prey, which is killed by constriction. Constriction involves holding the prey's rib cage closed when it exhales, so death comes by suffocation, not crushing. Boids that feed on such prey typically have heat-sensitive pits in their lips (similar to the pits in rattlesnakes) that allow them to "see" and track prey even in the dark. Those "primitive" species that feed on other reptiles lack these pits.