Pleural effusions result from imbalance between fluid formation and absorption in the pleural space. Transudative effusions form from increased hydrostatic pressure (CHF — most common cause, hepatic hydrothorax, nephrotic syndrome) or decreased oncotic pressure (hypoalbuminemia). The pleural membrane is intact — fluid is an ultrafiltrate of plasma. Exudative effusions result from increased capillary permeability due to inflammation (pneumonia, TB), malignancy (pleural metastases, mesothelioma), or lymphatic obstruction. Light's criteria differentiate transudative from exudative: an effusion is exudative if ANY ONE criterion is met — pleural protein/serum protein > 0.5, pleural LDH/serum LDH > 0.6, or pleural LDH > 2/3 upper limit of normal serum LDH. Light's criteria have 98% sensitivity for exudates but may misclassify transudates in patients on diuretics — apply serum-to-pleural albumin gradient (> 1.2 g/dL suggests transudate despite meeting Light's criteria).