This study is based on observational research in a Hospital Casualty Department, and focuses on the social processes involved in the categorisation of patients by staff members. It has two central themes. First, it points to the deep involvement of non-medical staff in important and diverse judgements over the nature of patients' conditions, and second, it suggests that medical categorisation in the Casualty Department setting can be seen to have many of the features of 'practical decision-making' described in studies of other kinds of organisations. Descriptions are offered of the importance of different staff types' judgemental activities at different stages of the patient passage. The early routing of the patient on a particular path through the Department is likely to influence such things as the time that passes before he sees a doctor, the number of casualty staff attending to him, and the physical facilities immediately at hand. Together with the verbal descriptions passed on to the doctor, this early routing may suggest a framework of expectation which will influence the doctor's own conception of the case. At a more overt level, nursing staff will frequently offer advice and openly exert influence on doctors' decisions - particularly where immigrants or inexperienced Casualty Officers are involved. Formally prescribed patterns of superior/subordinate relations are frequently undercut to some extent by informal social arrangements. In advancing a model of action in terms of practical decision-making, the thesis shows how casualty work leads staff through an endless series of 'commonsense' situations of choice. Staff members employ routine methods of investigation which enable them to recognise patients' conditions in terms of familiar categories, but use of such methods always involves interpretive judgements and is subject to modification according to the contingencies of particular situations.