Rebreathing Bag is also called as reservoir bag or breathing bag. It acts as a counter lung moving reciprocally with the patients lungs. It is ellipsoidal in shape, so that it can be grasped easily by one hand. The neck of the bag must be 22mm female fitting which connects with the breathing system. It is made of antistatic rubber with varying capacity from 0.5L to 7L (L-liter). Larger bags may be used in ventilator, while smaller bags are needed for paediatric anaesthesia. Reservoir bag should be soft to allow or maintain a low expiratory pressure. It helps accumulation of expired gas to some extent and thus allows re breathing, causes some economy of anaesthetic gases and vapours and prevents air dilution.