Situation awareness, and how systems can be designed to support it appropriately, have been a focus of study in dynamic, safety critical contexts such as aviation. The work reported here extends the study of situation awareness into the domain of emergency medical dispatch (EMD). The study was conducted in one of the largest ambulance services in the world. In this study, we encountered development and exploitation of situation awareness, particularly among the more senior EMD operators called allocators. In this paper we describe the notion of a 'mental picture' as an outcome of situation awareness, how an awareness of the situation is developed and maintained, the cues allocators attend to, and the difficulties they face in doing so. One of the key characteristics of ambulance control is that relatively routine behaviour is periodically interspersed with incidents that demand much higher levels of attention, but that the routine work must still be completed; operators exhibit contrasting levels of situation awareness for the different kinds of incidents. Our findings on situation awareness are related to those of others, particularly Endsley and Wickens. The observations and interviews enable us to propose high-level requirements for systems to support appropriate situation awareness, to enable EMD staff to complete their work effectively.