A headphone system comprising a physiological sensor and a method of fitting a headphone system comprising a physiological sensor to a user is provided. The headphone system comprises a headphone having a speaker, a physiological sensor configured to be positioned for measuring physiological data, and a processor connected to the physiological sensor to receive the measured physiological data and process the measured physiological data to output physiological information and a fitting parameter. An application program is associated with the headphone, and is configured to receive the physiological information and the fitting parameter. The application program may evaluate at least the fitting parameter to indicate to a user whether the headphone is properly positioned, and issue a first notification in dependence on the evaluation of the fitting parameter.