Asthma is an increasingly common disease. Those patients with the most severe disease consume a disproportionately high percentage of the healthcare budget. Sensitisation to a number of fungi including Aspergillus spp. is commonly found in atopic individuals. Sensitisation to fungi is associated with increased asthma medication usage, measured asthma severity, asthma admissions, intensive care admissions for asthma, respiratory arrest and asthma deaths. A number of mechanisms have been proposed for the ability of fungi to induce an allergic response. These include the wide spectrum of allergenic molecules released by fungi, especially those with inherent proteolytic activity. Volatile organic compounds have also been implicated but the evidence for this is less strong. Allergic bronchopulmonary aspergillosis (ABPA) is the archetypal allergic fungal disease whereby Aspergillus fumigatus colonises the airway in a sensitised individual, and is associated with the formation of Aspergillus-specific IgE and IgG. Severe asthma with fungal sensitivity (SAFS) describes a group of asthma patients with refractory asthma symptoms, sensitisation to one or more common fungi, but lacking the diagnostic criteria for ABPA (such as Aspergillus-specific IgG). The importance of this empirical categorization lies in the response to anti-fungal therapy. The FAST trial showed that such patients had both clinically and statistically significant improvements in quality of life, rhinitis scores, and reductions in serum IgE in response to itraconazole when compared to placebo.