Various proposed definitions of occupational asthma are presented. Each includes a definition for asthma and a characterization of its relation to the workplace. Features used to establish the clinical diagnosis of occupational asthma are also defined; these include the presence of air flow limitation, hyperresponsiveness of airways, potential for or presence of atopy, work relatedness of air flow limitation, and sensitization to a specific agent. Methodologic issues in epidemiologic studies of asthma are discussed, including the approach, outcome measurements, exposure measurements, and target populations. The epidemiology of environmental and occupational asthma was briefly reviewed. The topics discussed included estimates of prevalence, distribution and determinants in work force based studies, and wheezing and occupational exposure in community based studies. Screening and surveillance and future initiatives for occupational asthma are also discussed.