Intra-abdominal infections represent a wide variety of pathological conditions that involve lesions of all the intra-abdominal organs. They include both inflammation of single organs and any sort of peritonitis (primary, secondary, tertiary), where the severity of the disease often depends on the extension of the inflammation (local or diffuse peritonitis). They also include intraperitoneal, retroperitoneal and parenchymal abscesses. the aim of this article is to analyze the current definitions and classifications of intra-abdominal infections.