Abstract Pulmonary function testing is one of the basic tools for evaluating a patient's respiratory status. Respirometers are used for an objective assessment of pulmonary function. They can measure the mechanical function of lungs, chest wall and respiratory muscles by recording volume, flow and pressure changes during expiratory or inspiratory manoeuvres. Many of these devices are commonly used for monitoring and evaluating patients in anaesthesia and intensive care environments. Some of these instruments are used more commonly for investigation and research whereas others are used in routine clinical practice. After a brief discussion on the physics of gas flow, this article describes the basic principle of commonly used respirometers and their practical applications, advantages and disadvantages. However, more attention has been paid to those instruments used in day to day clinical practice especially in anaesthetic practice. A detailed discussion on volume measuring devices such as the vitalograph and Wright's respirometer, and flow measuring devices sych as the pneumotachograph and peak flow meter has been included. Electronic modifications and computerized systems of some of the conventional respirometers are discussed. Two other important flow measuring devices (hot wire flowmeter and ultrasonic flowmeter) are also discussed.