The hallux or big toe is the biological name for digit I. In humans and non-human primates, the hallux is the largest toe on the foot.

The human big toe has limited grasping ability. Chimpanzees and many other non-human primates, who lack opposable thumbs, have an opposable hallux.

In birds with a anisodactyl or heterodactyl feet, the hallux is opposed or directed backwards and allows for grasping and perching.