Pleural effusion affects 1.5 million patients in the United States each year. New effusions require expedited investigation because treatments range from common medical therapies to invasive surgical procedures. The leading causes of pleural effusion in adults are heart failure, infection, malignancy, and pulmonary embolism. The patient's history and physical examination should guide evaluation. Small bilateral effusions in patients with decompensated heart failure, cirrhosis, or kidney failure are likely transudative and do not require diagnostic thoracentesis. In contrast, pleural effusion in the setting of pneumonia (parapneumonic effusion) may require additional testing. Multiple guidelines recommend early use of point-of-care ultrasound in addition to chest radiography to evaluate the pleural space. Chest radiography is helpful in determining laterality and detecting moderate to large pleural effusions, whereas ultrasonography can detect small effusions and features that could indicate complicated effusion (i.e., infection of the pleural space) and malignancy. Point-of-care ultrasound should also guide thoracentesis because it reduces complications. Computed tomography of the chest can exclude other causes of dyspnea and suggest complicated parapneumonic or malignant effusion. When diagnostic thoracentesis is indicated, Light's criteria can help differentiate exudates from transudates. Pleural aspirate should routinely be evaluated using Gram stain, cell count with differential, culture, cytology, protein, l-lactate dehydrogenase, and pH levels. Additional assessments should be individualized, such as tuberculosis testing in high-prevalence regions. Parapneumonic effusions are the most common cause of exudates. A pH level less than 7.2 is indicative of complicated parapneumonic effusion and warrants prompt consultation for catheter or chest tube drainage, possible tissue plasminogen activator/deoxyribonuclease therapy, or thoracoscopy. Malignant effusions are another common cause of exudative effusions, with recurrent effusions having a poor prognosis.