Intra-abdominal infections (IAIs) are a common cause of sepsis, and frequently occur in intensive care unit (ICU) patients. IAIs include many diagnoses, including peritonitis, cholangitis, diverticulitis, pancreatitis, abdominal abscess, intestinal perforation, abdominal trauma, and pelvic inflammatory disease. IAIs are the second most common cause of infectious morbidity and mortality in the ICU after pneumonia. IAIs are also the second most common cause of sepsis in critically ill patients, and affect approximately 5% of ICU patients. Mortality with IAI in ICU patients ranges from 5 to 50%, with the wide variability related to the specific IAI present, associated patient comorbidities, severity of illness, and organ dysfunction and failures. It is important to have a comprehensive understanding of IAIs as potential causes of life-threatening infections in ICU patients to provide the best diagnostic and therapeutic care for optimal patient outcomes in the ICU.