Pleural fluid serves a physiologic function in respiration, while also being a useful measure to diagnose and assess disease, trauma, and other abnormalities. A brief review of the anatomy and physiology of normal pleural fluid gives a point of reference for assessing the causes of abnormal pleural fluid collections and pleural effusions.  [1][2][3] The Light criteria is a useful way to differentiate between transudate and exudate, which can then be further evaluated with lab tests and in the context of the clinical presentation of the patient. Evaluation of pleural fluid can be used to determine the cause of pleural effusion and help guide the treatment of the underlying cause.