The collapse of part or whole of the lung is called as Atelectasis. This will occur due to pressure on outside of lung or due to blockage of air passages in bronchioles. It results in reduced or absence of gas exchange. In this the alveoli are deflated down to little or even no volume. Atelectasis is not similar to the disease of another type of collapsed lung called pneumothorax, which is caused due to escape of air from the lungs. Then the air fills in the space outside of the lung, which is between the lung and chest wall. It is diagnosed by chest X-ray. CT scan, oximetry, bronchoscopy and ultrasound of thorax are used in the diagnosis of the disease. Atelectasis is common for the people who are in hospital after surgery. Treatments include breathing exercises, medication like bronchodilator; oxygen therapy for supportive care is recommended.