Studies of performance in medicine are often based on observation. Videotape provides a valuable tool for recording events from both real environments and simulators. When analysing observational data it is important that robust tools are used, particularly when investigating non-technical (cognitive and social) skills. This paper describes the method used to identify the key non-technical skills required in anaesthesia and to develop a behavioural marker system for their measurement. A prototype taxonomy was designed on the basis of a literature review; an examination of existing marker systems; cognitive task analysis interviews; an iterative development process involving workshops; and cross-checking in theatre. The resulting anaesthetists non-technical skills (ANTS) system comprises four skill categories (task management, team working, situation awareness, and decision making) that divide into 15 elements, each with example behaviours. Preliminary evaluation using ratings of videotaped scenarios indicated that the skills were observable and could be rated with reasonable agreement.