emotional. In safety critical industries, not being able to assess your status has safety implications. In ATM fatigue is a major factor that can affect controller’s performance and safe conduct. This paper explores an approach to developing a prototypical Fatigue Index Tool (FIT) to assess factors relevant to fatigue and their impact on air traffic controllers. Preliminary research has evaluated sources of literature and identified key common factors in shift workers related to fatigue. The existence and use of countermeasures against fatigue from the literature was reviewed and integrated within FIT. A review of the factors identified from the literature was undertaken by controllers at EUROCONTROL’s EXPERIMENTAL CENTRE (EEC) and a further validation exercise with operational staff at the Air Navigation Service Provider (ANSP) “CRNA NORD (Athis Mons)”. The presentation of the prototype FIT was refined, taking account of the suggestions provided by the Athis Mons controllers