Occupational asthma is a common disorder that may be caused by several hundred agents and has a variety of pathogenetic mechanisms. Level of exposure is an important risk factor, and reduction of exposure is the only certain method of prevention. Atopy and smoking are further risk factors for IgE-mediated asthma but have not been found to increase risk in forms of the disorder that have other mechanisms. The key to diagnosis is a low threshold of suspicion; several investigative procedures can be used to confirm the diagnosis. Many patients suffer from continued asthma despite cessation of exposure; early diagnosis and early removal from exposure are the most important factors for improving the long-term outcome.