A humidifier (10; 30) for humidifying a flow of breathable gas to be delivered to a patient, the humidifier comprising: a humidifier body configured to retain a body of liquid having a predetermined maximum volume (V max), the humidifier body comprising: a first chamber having a first chamber inlet configured to receive the flow of breathable gas, and a second chamber in communication with the first chamber (16; 74) through a passage, the second chamber (14; 76) being structured to contain the predetermined maximum volume of liquid when the humidifier body is in a normal upright, operating position, the second chamber (14; 76) comprising a second chamber outlet configured to deliver the flow of breathable gas with added humidity, wherein the first chamber (16; 74), the second chamber (14; 76), and the passage (20; 50) are arranged such that when liquid is contained in the second chamber (14; 76) in the upright, normal operating position, the possibility of liquid flowing from the second chamber (14, 76) through the passage (20; 50) is reduced or prevented, and any liquid that flows from the second chamber (14; 76) and through the passage (20; 50) is collected in the first chamber (16; 74) such that liquid is discouraged or prevented from spilling back from the first chamber inlet (22; 32) when the humidifier body is inadvertently rotated from the upright, normal operating position to a non-upright position. a humidifier body configured to retain a body of liquid having a predetermined maximum volume (V max), the humidifier body comprising: a first chamber having a first chamber inlet configured to receive the flow of breathable gas, and a second chamber in communication with the first chamber (16; 74) through a passage, the second chamber (14; 76) being structured to contain the predetermined maximum volume of liquid when the humidifier body is in a normal upright, operating position, the second chamber (14; 76) comprising a second chamber outlet configured to deliver the flow of breathable gas with added humidity, a first chamber having a first chamber inlet configured to receive the flow of breathable gas, and a second chamber in communication with the first chamber (16; 74) through a passage, the second chamber (14; 76) being structured to contain the predetermined maximum volume of liquid when the humidifier body is in a normal upright, operating position, the second chamber (14; 76) comprising a second chamber outlet configured to deliver the flow of breathable gas with added humidity, wherein the first chamber (16; 74), the second chamber (14; 76), and the passage (20; 50) are arranged such that when liquid is contained in the second chamber (14; 76) in the upright, normal operating position, the possibility of liquid flowing from the second chamber (14, 76) through the passage (20; 50) is reduced or prevented, and any liquid that flows from the second chamber (14; 76) and through the passage (20; 50) is collected in the first chamber (16; 74) such that liquid is discouraged or prevented from spilling back from the first chamber inlet (22; 32) when the humidifier body is inadvertently rotated from the upright, normal operating position to a non-upright position.