An in-line humidifier is disposed along the passageway of a breathing system. The humidifier includes a humidification chamber having a fluid inlet, a fluid outlet, and a reservoir. The humidifier includes a liquid trap which includes a chamber, a fluid inlet in communication with the liquid trap chamber, and a fluid outlet in communication with the liquid trap chamber. The liquid trap is constructed and arranged to discourage liquid from back-flowing from the humidifier to a source of gas.