Septic peritonitis is an inflammatory condition of the peritoneum that occurs secondary to microbial contamination. This clinically important condition has a wide variety of clinical courses as well as high morbidity and mortality due to secondary multiorgan dysfunction. This article reviews the etiology and pathophysiology of this condition and its diagnosis in small animals; a companion article addresses treatment and prognosis. The disease is Monarch Disease Ontology entry MONDO_0005195 (septic peritonitis).