'Accidents appear to be the result of highly complex coincidences which could rarely be foreseen by those involved ... accidents do not occur because people gamble and lose, they occur because people do not believe that the accident that is about to occur, is at all possible.' Wagenaar and Groeneweg (1987) The air traffic management system is considered a high reliability, low risk activity. It is also seen as inherently safe despite the tightly coupled nature of the activities involved. Tightly coupled systems are characterised by time-dependant processes in which planned and unplanned interactions occur quickly and are also associated with the co-ordination of activities which must occur in a pre-defined sequence adhering to a set of strict rules and procedures. They are also very precise and vulnerable to unexpected events in which the activities must be undertaken correctly or not at all. All these characteristics can be found in the air traffic management (ATM) system and in particular in their 'team related' activities. In these systems there is little opportunity to improvise when things go wrong, and recovery is very time dependant. However there is almost no existing information about the types of error found in the ATM system and the recognition, detection and recovery from the errors made. Therefore in order to obtain information to better understand these issues, three distinct but complimentary studies were undertaken. In the first study a technique to identify both the cognitive and contextual factors of erroneous behaviour was developed. Several incidents were analysed using this technique, revealing that planning and decision making was the most problematic cognitive area and team difficulties accounted for the majority of the contextual conditions associated with the errors. Results also indicated the importance of the relationship of all the members of the air traffic management team - including pilots - and their role in the detection, recovery and management of the situation. This resulted in the development of a new technique - the Joint Error Detection of Incidents - JEDI - to be used in the exploration of the chronology of occurrences. In the second study controllers were questioned with regard to their attitudes towards error, safety and teamwork. Findings from this work suggested the majority of controllers were aware of the team interaction issues, but were more concerned with their relationship with other teams, particularly in emergencies and unusual situations. In the third, a study was undertaken to record the erroneous activities and outstanding events during ATM simulation sessions. The results of these activities revealed that rather than controllers making errors, they were able, in the majority of the cases, to assess the outstanding event in relation to their professional knowledge and experience. These events have been termed 'Expert Judgement Deviations' and consist of behaviour which enable the system to continue in its tightly coupled structure. This study was also able to determine the differences between errors and violations and again indicated that errors were mostly attributable to failures in planning and decision making. This study also highlighted the importance of team issues but demonstrated that the team, as defined in the literature, was probably not a suitable concept in such an environment. From the insights gained in this research it was argued that, although the team was an important concept, the separation of aircraft undertaken by controllers was an individual activity. However it was also realised that without the co-operation and co-ordination between controllers the overall goals of safety would be compromised. Therefore the term 'dynamic sets' was introduced and used to describe the majority of the controllers work in the Air Traffic Management system.