Work-related asthma (WRA) is asthma that is attributable to, or is made worse by, environmental exposures in the workplace. WRA has become the most prevalent occupational lung disease in developed countries, is more common than is generally recognized, and can be severe and disabling. Identification of workplace exposures causing and/or aggravating the asthma, and appropriate control or cessation of these exposures can often lead to reduction or even complete elimination of symptoms and disability. This depends on timely recognition and diagnosis of WRA. In this review, the diagnostic evaluation has been organized in a stepwise fashion to make it more practical for primary care physicians as well as physicians specializing in occupational diseases and asthma. WRA merits more widespread attention among clinicians, labor and management health and safety specialists, researchers, health care organizations, public health policy makers, industrial hygienists, and others interested in disease prevention. Am. J. Ind. Med. 37:121–141, 2000. © 2000 Wiley-Liss, Inc.