The term atelectasis describes a state of collapsed and non-aerated region of the lung parenchyma, which is otherwise normal. This pathological condition is usually associated with several pulmonary and chest disorders and represents a manifestation of the underlying disease, not a disease per se. Atelectasis may occur in three ways: (i) airway obstruction; (ii) compression of parenchyma by extrathoracic, intrathoracic, chest wall processes; and (iii) increased surface tension in alveoli and bronchioli. Chest radiographs using both the anterior-posterior and lateral projections are mandatory to document the presence of atelectasis. Differentiation from lobar consolidation may be a clinical dilemma. The treatment of atelectasis varies depending on duration and severity of the causal disease from chest physiotherapy to postural drainage, bronchodilator and anti-inflammatory therapy. Persistent mucous plugs should be removed by bronchoscopy.