Severe asthma is often refractory to standard treatments and presents real problems of management. It necessitates rigorous clinical procedures to identify the aggravating factors. Allergic factors probably play an important role but are often ignored; atopy is more often associated with mild or moderate asthma. The prevalence of severe refractory asthma is not known with any precision, in view of the lack of a consensus definition. Few studies have examined the role of allergic factors. From a pathophysiologic perspective, genetic factors are probably responsible for greater severity, and allergic response plays an important role in some severe asthma phenotypes. From an environmental perspective, some airborne allergens are associated with the refractory character of the disease. Molds such as Alternaria are, for example, implicated in the onset of asthma attacks, as are some occupational exposure. Better knowledge of the pathophysiology of refractory asthma and its allergic dimensions have made it possible to use new treatment agents, such as omalizumab. New treatment targets are being discovered and evaluated in this domain.