A compliance monitoring system and method in which a modular compliance monitor is selectively disposed in-line in a patient circuit of a pressure support system. The monitor includes a housing having an inlet and an outlet, each of which is selectively connectable to the patient circuit so that the flow of gas in the patient circuit also passes through a passage is provided through the housing. A sensor operatively coupled to the passage detects a parameter indicative of patient breathing. A processor disposed in the housing and receives the output of the sensor and determines, from the output of the sensor, whether the patient is breathing into the patient circuit, and outputs compliance data based on this determination.