When a diagnosis of acute pancreatitis (AP) is made, fundamental medical treatment consisting of fasting, intravenous (IV) fluid replacement, and analgesics with a close monitoring of vital signs should be immediately started. In parallel with fundamental medical treatment, assessment of severity based on clinical signs, blood test, urinalysis and imaging tests should be performed to determine the way of treatment for each patient. A repeat evaluation of severity is important since the condition is unstable especially in the early stage of AP. At the time of initial diagnosis, the etiology should be investigated by means of blood test, urinalysis and diagnostic imaging. If a biliary pancreatitis accompanied with acute cholangitis or biliary stasis is diagnosed or suspected, an early endoscopic retrograde cholangiopancreatography with or without endoscopic sphincterotomy (ERCP/ES) is recommended in addition to the fundamental medical treatment. In mild cases, the fundamental medical treatment should be continued until clinical symptom is subsided with normal laboratory data. In cases with severe acute pancreatitis (SAP) referral should be considered to medical centers experienced in the treatment of SAP, and intensive care is recommended for preventing both organ failures and infectious complications. Hemodynamic stabilization with vigorous fluid resuscitation, respiratory support and antibiotics are the major parts of intensive care in the early period of SAP. Continuous hemodiafiltration (CHDF) and continuous regional arterial infusion (CRAI) of protease inhibitor and/or antibiotics may be effective to improve pathophysiology of AP especially in the early stage of the disease. In the late stage of AP, infectious complications are critical. If an infectious complication is suspected based on clinical signs, blood test and imaging, a fine needle aspiration (FNA) is recommended to establish a diagnosis. The accuracy of FNA is reported to be 89 ~ 100%. For patients with sterile pancreatitis, non-surgical treatment should be indicated. For patients with infected pancreatic necrosis, therapeutic intervention either by percutaneous, endoscopic, laparoscopic or surgical approach are indicated. The most preferred surgical intervention is necrosectomy, however, non-surgical treatment with antibiotics is still the treatment of choice if the general condition is stable. Necrosectomy should be performed as late as possible. For patients with pancreatic abscess, drainage is recommended.