Occupational asthma is one of the most frequent work-related diseases and may represent between 2% and 6% of all cases of asthma. It is defined as asthma causally and specifically related to exposure to airborne dusts, gases, vapors, or fumes in the working environment. Because it may cause long-lasting disability, it is important to properly identify affected workers and to withdraw them from exposure to the sensitizing agent as soon as possible. Although the history is the clue to the diagnosis, it is not sensitive or specific. The diagnosis should be confirmed by objective means, essentially by monitoring of peak expiratory flow and nonallergic bronchial responsiveness or by specific inhalation challenges. In this article the author reviews the investigation of occupational asthma.