The workplace can trigger or induce asthma and cause the onset of different types of work-related asthma. Analysis of previous definitions of occupational asthma (OA) led to the conclusion that evidence of a direct causal relationship between workplace exposure and the development of asthma remains the key element for defining OA. Based on clinical features and pathophysiological mechanisms, the following conditions should be distinguished in the spectrum of work-related asthma: 1) immunological OA characterised by a latency period necessary to acquire immunologically induced sensitisation; 2) nonimmunological OA characterised by the rapid onset of asthma following single or multiple exposures to high concentrations of irritant compounds; 3) work-related asthma defined by exacerbation of symptoms in workers with pre-existing or coincident asthma; and 4) variant syndromes including eosinophilic bronchitis, potroom asthma, and asthma-like disorders caused by organic dusts. The issues and controversies relating to this approach are critically reviewed in order to stimulate the consensus development of operational definitions of work-related asthma.