Asthma is a common chronic illness characterized by episodes of reversible airflow obstruction. A cornerstone of asthma management is identifying and avoiding agents that cause bronchospasm. The workplace is an important potential source of respirable exposures that can cause or trigger asthma. Identification of an occupational factor in asthma is important: early diagnosis and removal of the worker from the exposure is associated with improved prognosis; the diagnosis of occupational asthma may lead to compensation for work-related impairment and disability; and the diagnosis of occupational asthma is a Sentinel Health Event with implications for public health and prevention. In this article, we review specific causes of occupational asthma and general settings in which an occupational factor should be suspected and explored as part of the management of the worker with asthma. We also review specific and simple elements of history and pulmonary function testing that can be easily assessed by most health care practitioners and may be sufficient to establish a diagnosis of occupational asthma. Finally, we review the medical-legal implications of occupational asthma.