Exudate is the leakage of fluid, often due to inflammation, into a confined cavity in the body, such as a pleural effusion. The color of exudate is usually straw yellow, slightly cloudy, and the specific gravity is a little larger than that of leaking fluid. Exudate is commonly caused by exudate from pleurisy or exudate from subdiaphragmatic infection, but also from pneumothorax and exudate from chest surgery. Treatment of exudate, once it is diagnosed or suspected to be an exudate, must be treated by puncture. Exudate must be differentiated from leaky fluid because the two are treated differently. The differential diagnosis between exudate and leaky fluid can be made mainly by qualitative and quantitative protein tests, quantitative glucose tests or cell counts, the cause of the fluid production, and the patient’s medical history.