Pleural effusions occur when fluid accumulates between the visceral and parietal pleura in the chest cavity, preventing the lung from expanding fully during inspiration. The reduction in lung volume, depression of the diaphragm and reduced chest wall compliance cause dyspnoea, and occasionally pain or cough. Pleural effusion is a complex problem caused by a variety of conditions requiring different treatment depending on the underlying diagnosis. This article discusses the causes and treatment of pleural effusions, referencing guidelines produced by the British Thoracic Society.