Occupational asthma (OA) is steadily emerging as the principal cause of respiratory disease due to the workplace environment. One of the key means to ascertain diagnosis of OA is specific inhalation challenge (SIC) with occupational agents. This review: 1) describes the methodology of SIC, with a special emphasis on procedures aimed at increasing the safety and validity of these tests; and 2) outlines the roles of SIC in the diagnosis of OA in clinical and medicolegal assessment, epidemiological studies, surveillance programmes and the investigation of the pathophysiological mechanisms of asthma and OA. We discuss areas of future development, including the development of apparatus which allows exposure of subjects to low and stable concentrations of the occupational agent and the assessment of preventive procedures.