Whereas there are several validated systems for reporting postoperative complications, there are only a few and not prospectively validated systems for reporting intraoperative complications. The investigators developed a definition and CLASSification for Intraoperative Complications CLASSIC within a Delphi study involving international interdisciplinary experts. As both surgery and anaesthesia may be involved in complications in the perioperative period, all patient-related intraoperative complications occurring between skin incision and closure are considered in this reporting system. CLASSIC was updated to ClassIntra® v1·0, by encompassing five severity grades to align with the validated Clavien-Dindo Classification for grading postoperative complications.

The aim of this international multicentre observational cohort study is to assess the validity and practicability of this newly derived and updated classification system ClassIntra® in patient undergoing an in-hospital surgical procedure. Providing an easy applicable and internationally validated classification system is a contribution to improve quality of health care and patient safety.

Inclusion Criteria:

• All in-hospital patients (consecutive or random sample) undergoing surgery in the operating room with anaesthesia involvement

Exclusion Criteria:

• Outpatients (patients undergoing one-day-surgery) (with and without anaesthesia-involvement)
• Procedures without anaesthesia-involvement (in- or out-patient)
• ASA risk classification (ASA) VI patients (brain-death organ-donor)

All in-hospital patients (consecutive or random sample) undergoing surgery in the operating room with anaesthesia involvement