This document discusses pleural effusions, including their mechanism, normal composition, differentiation between transudative and exudative effusions, and causes. It provides details on parapneumonic effusions and empyema, including classification and treatment approaches. Specific conditions discussed in detail include tuberculous pleural effusions, effusions in HIV patients, chylothorax, pseudochylothorax, and malignant pleural effusions. Useful pleural fluid tests are also summarized.