The key to effective evaluation of pleural effusions lies in an understanding of the dynamic nature of its formation and alteration. Trying to fit a specimen neatly into a distinct diagnostic category will not only lead to frustration, but is often unnecessary or incorrect. Combined etiologies are common, and although the possibilities are infinite, certain patterns often present themselves: inflammation frequently complicates congestive failure and neoplasms, hemorrhage is common in neoplastic and chylous effusions, neoplastic effusions may cause transudation from venous obstruction or pericardial stricture, and sepsis is frequently secondary to traumatic effusions. In the presence of atypical findings and the establishment of one cause of pleural effusion, one cannot rule out other operative conditions.