The circle system is the most common design for anesthesia machines. Its components include the one-way valves on the expiratory and inspiratory limbs, corrugated breathing tubes, Y piece, APL valve, Reservoir Bag, CO2 cannister. The circle system can be utilized as either a closed or semi-closed system depending on the mode of ventilation and the setting on the APL valve. Thus, rebreathing is either minimal or non-existent. This circuit can be used in people of nearly all ages, but certain considerations must be taken into for neonates and infants due to their inherent physiological differences from adults. The circle system is deemed cost effective from the standpoint of volatile anesthetic utilization and can conserve heat within the breathing circuit. However, the circle system increases resistance and thus increases the work of breathing. With the complex design of the typical circle system machines troubleshooting becomes more cumbersome.