The Air Traffic Management (ATM) system is a ‘joint cognitive system’ of people, teams and artefacts (e.g., radar consoles, procedures, automated support functions, alerting services) that adapts to the challenges and demands posed by unfamiliar situations and new technologies. In a joint cognitive system, practitioner activities are understandable in relation to the properties of the environment in which they work; for instance, conflict resolution strategies are affected by traffic constraints, availability of job aids, team composition, collaboration with flight crews and regulations. In turn, a work environment can be understood in terms of what it demands and affords to the people at the sharp end. This chapter presents an overview of the work environment of air traffic controllers (ATCOs) in order to provide a basis for understanding human performance challenges. Very often, the demand for increased throughput has stretched ATM resources and capabilities to a point where accident rates may be difficult to control.