Complicated intra-abdominal infections are frequently associated with poor prognoses and high morbidity and mortality rates.Despite advances in diagnosis, surgery, and antimicrobial therapy, mortality rates associated with complicated intra-abdominal infections remain exceedingly high.In order to describe the clinical, microbiological, and management-related profiles of both community-acquired and healthcare-acquired complicated intra-abdominal infections (IAIs), the World Society of Emergency Surgery (WSES), in collaboration with the Surgical Infections Society of Europe (SIS-E) and other prominent European surgical societies, has designed the CIAO study.The CIAO study is a multicenter, observational study and will be carried out in various surgical departments throughout Europe. The study will include patients undergoing surgery or interventional drainage for complicated IAI.