Bacterial septic peritonitis is a serious condition that requires immediate treatment. The pathogenesis is complex, and the list of diagnostic differentials is extensive. The keys to successful treatment are early recognition of the condition and elimination of the causative organism. Multiple options for draining the peritoneal cavity exist, and further studies are necessary to establish specific, evidence-based guidelines. The prognosis is generally guarded in dogs and cats. Much depends on whether the patient develops concurrent sepsis, systemic inflammatory response syndrome, or multiple organ dysfunction syndrome.