Occupational asthma is the most common occupational respiratory disease in many developed countries as well as in Singapore. About 4% to 9% of adult asthmatics may have occupational asthma. The condition is still under-diagnosed and under-reported. Asking an adult asthma patient whether his or her symptoms improve when away from work and worsen during periods at work is important in detecting potential cases of occupational asthma. Serial peak expiratory flow rate (PEFR) monitoring during periods at work and away from work is useful for providing objective documentation of work-relatedness. Specific challenge testing is carried out to confirm a specific causative agent where a new agent is suspected, where there are multiple agents involved and it is important to establish the exact agent, and when it is not possible to carry out serial PEFR monitoring. Early diagnosis and removal from further exposure to the causative agent in the workplace will benefit the patient with occupational asthma. Preventive measures are important to protect other workers at risk.