A wearable physiological sensor has a housing and a gas-permeable support structure carried by the housing, which contacts the skin of the subject. An air space is provided between the support structure and the housing. Movement of the support structure relative to the housing is sensed. This provides a sensor which is comfortable for the subject and provides good sensitivity in that motion being detected (e.g. an arterial pulse) only needs to impart kinetic energy to the support structure, with a relatively low inertia.