A reservoir configured to retain a volume of liquid for use in an apparatus for humidifying a flow of pressurised air comprises a base portion and a lid portion. The reservoir may be configured to improve its level of thermal contact to the heater plate using the flow of pressurised air. The reservoir may be configured to improve thermal contact between the reservoir and the heater plate by pre-compression upon engagement of the reservoir with the humidifier. The reservoir may comprise a removable intermediate portion, which may include the inlet tube and/or the outlet tube, for improved access for cleaning. The reservoir may also be configured to prevent overfilling. Overfill prevention features in the reservoir may include defined flow egress paths and/or formation of air locks.