The Eustachian tube is a part of the ear, an evolutionary descendant of the gills in fish. The tube links the pharynx to the middle ear. In adults the Eustachian tube is approximately 3.5 cm long.

Some modern medical books call this the pharyngotympanic tube.

Normally the Eustachian tube is closed, but it can open to let a small amount of air through to equalize the pressure between the middle ear and the atmosphere. When this happens we hear a small pop, an event familiar to airplane travelers or drivers in mountainous regions. Yawning or swallowing can pull on muscles in the neck, cause the tube to open. Some people are born with the ability to contract just these muscles voluntarily, similar to people who can wiggle their ears. Without this airway, the inner ear would be isolated from the atmosphere, and could be easily damaged by pressure changes.

The Eustachian tube also drains mucus from the middle ear. Upper airway infections or allergies can cause the Eustachian tube to become swollen, trapping bacteria and causing ear infections. Earache is more common in children because the tube is more horizontal, making the movement of fluid harder.

When descending in an aircraft, increase in atmospheric pressure can lock the Eustachian tube. The tube can be reopened by the Valsalva maneuver or politzerization.

It is named after the 16th century anatomist Eustachius.