The term atelectasis is derived from the Greek words ateles and ektasis, which mean incomplete expansion. Atelectasis is defined as diminished volume affecting all or part of a lung. Pulmonary atelectasis is one of the most commonly encountered abnormalities in chest radiology findings. Recognizing an abnormality due to atelectasis on chest x-ray films can be crucial to understanding the underlying pathology. Several types of atelectasis exist; each has a characteristic radiographic pattern and etiology. Atelectasis is divided physiologically into obstructive and non-obstructive causes.