Postoperative pulmonary complications (PPCs) remain a significant cause of morbidity and mortality in the surgical population. These complications encompass a spectrum of conditions, including hypoventilation, atelectasis, pneumonia (including aspiration), pulmonary embolism (PE), acute respiratory distress syndrome (ARDS), and less common but clinically important events such as negative pressure pulmonary edema. The perioperative period is uniquely vulnerable to respiratory complications due to anesthetic effects, surgical trauma, patient comorbidities, and postoperative pain or immobility. Hypoventilation can arise from pre-existing disorders such as obstructive sleep apnea and obesity hypoventilation syndrome, among others, compounded by residual sedation, incomplete reversal of neuromuscular blockade, possible central nervous system events, or metabolic disturbances such as hypoglycemia and electrolyte abnormalities. Atelectasis is ubiquitous after general anesthesia and sets the stage for pneumonia and impaired oxygenation. Pneumonia and aspiration represent major contributors to postoperative morbidity, particularly in frail or critically ill patients. PE, though not strictly a pulmonary disease, remains a major postoperative threat. ARDS may complicate major surgeries, transfusions, or thoracic procedures, representing severe inflammatory lung injury with elevated morbidity and mortality. Negative pressure pulmonary edema, though rare, can occur acutely after airway obstruction such as laryngospasm, particularly in young, healthy patients. This chapter reviews the pathophysiology, risk factors, perioperative presentation, and management strategies for these complications, with attention to diagnosis and treatment in the immediate postoperative period. For internists and anesthesiologists alike, recognition and prevention of PPCs remain central to improving perioperative outcomes.