Increased intra-abdominal pressure (IAP) results in intra-abdominal hypertension (IAH) and abdominal compartment syndrome (ACS). ACS is a condition that causes multiorgan disorders such as respiratory, circulatory, and renal function due to a rapid increase in IAP, and is fatal if decompression is not performed promptly. The main causes of acute elevation of IAP include intra-abdominal or retroperitoneal bleeding, intestinal or retroperitoneal edema, packing by damage control surgery, and ascites. Diagnosis is obtained by measuring intra-bladder pressure and the presence or absence of organ dysfunction, and treatment is appropriate fluid infusion, medical treatment to reduce IAP, surgical decompression and open abdomen management. Initially, most reports of ACS were on surgical diseases; however, in the ICU, it has been reported that IAH also occurs relatively frequently in medical diseases. In addition, IAP has been identified as a predictor of mortality and multiple organ dysfunction. The definition of IAH/ACS and clinical practice guidelines were published in 2006 and revised in 2013, and are currently recognized as important pathophysiological conditions mainly in the critical care. This section describes the history, definition, pathophysiology, diagnosis, and treatment of ACS.