In a specific inhalation challenge (SIC) test the patient inhales an occupational agent in controlled environment and the subsequent asthmatic reaction is monitored. SIC is considered as the reference standard when confirming the diagnosis of sensitizer-induced occupational asthma. However, SIC is not always needed for the diagnosis; in many cases the causal relationship between an occupational agent and asthma can be shown also with serial peak flow measurements and specific immunologic testing. SIC is invaluable in identifying new occupational airway sensitizers. This is essential for preventing occupational asthma in the future.