The post-anesthesia care unit (PACU) is a cornerstone of perioperative medicine, providing intensive monitoring and prompt intervention for patients recovering from anesthesia and surgery. As patients emerge from general, regional, or monitored anesthesia care, they exhibit varying degrees of residual anesthetic effect, physiological instability, and potential complications. The PACU functions as a critical transitional environment between the operating room and subsequent care settings, whether outpatient recovery, hospital ward, step-down, or intensive care unit. Within this setting, structured observation, early recognition of complications, and adherence to standardized discharge criteria are essential to ensuring patient safety and optimizing postoperative outcomes. PACU organization requires appropriate staffing, monitoring equipment, and standardized protocols. Patient assessment in the PACU follows an airway, breathing, circulation, neurologic, and pain-focused approach. Recovery is generally divided into Phase I (immediate post-anesthesia recovery, emphasizing physiological stabilization) and Phase II (intermediate recovery, focusing on readiness for discharge from the PACU or same-day surgery unit). Discharge criteria such as the Aldrete Score or modified post-anesthesia discharge scoring system (PADSS) ensure that patients are safely transitioned to the next level of care. This chapter reviews the structure of the PACU, core principles of patient assessment, phases of care, common complications, discharge readiness, and systems-based aspects, including staffing and handoffs. For internists and perioperative clinicians, understanding PACU processes enhances interdisciplinary communication and clarifies postoperative pathways, helping contextualize early complications that often require internist input after PACU transfer.