The chest wall comprising both rib cage and abdomen moves during breathing, and when suitably calibrated this motion can be used to monitor pulmonary ventilation without the need for face-masks or mouthpieces. Such non-invasive methods of studying breathing are likely to become more widely used in the future, especially in the investigation of respiratory control and sleep-disordered breathing. This review describes the physiological basis of these measurements and examines the practical and theoretical limitations of calibration techniques based on the two degrees-of-freedom model of chest-wall motion.