Background: Surgical outcomes are traditionally evaluated by post-operative data such as histopathology and morbidity. Although these outcomes are reported using accepted systems, their ability to influence operative performance is limited by their retrospective application. Interest in direct measurement of intraoperative events is growing but no available systems applicable to routine practice exist. We aimed to develop a structured, practical method to report intraoperative adverse events enacted during minimal access surgical procedures. Methods: A structured mixed methodology approach was adopted. Current intraoperative adverse event reporting practices and desirable system characteristics were sought through a survey of the EAES executive. The observational clinical human reliability analysis method was applied to a series of laparoscopic total mesorectal excision (TME) case videos to identify intraoperative adverse events. In keeping with survey results, observed events were further categorised into non-consequential and consequential, which were further subdivided into four levels based upon the principle of therapy required to correct the event. A second survey phase explored usability, acceptability, face and content validity of the novel classification. Results: 217 h of TME surgery were analysed to develop and continually refine the five-point hierarchical structure. 34 EAES expert surgeons (69%) responded. The lack of an accepted system was the main barrier to routine reporting. Simplicity, reproducibility and clinical utility were identified as essential requirements. The observed distribution of intraoperative adverse events was 60.1% grade I (non-consequential), 37.1% grade II (minor corrective action), 2.4% grade III (major correction or change in post-operative care) and 0.1% grade IV (life threatening). 84% agreed with the proposed classification (Likert scale 4.04) and 92% felt it was applicable to their practice and incorporated all desirable characteristics. Conclusion: A clinically applicable intraoperative adverse event classification, which is acceptable to expert surgeons, is reported and complements the objective assessment of minimal access surgical performance.