Severe asthma phenotypes have been associated with a number of allergic and nonallergic factors. Specific allergens, such as cockroach and Alternaria, as well as mould sensitivity, are associated with severe exacerbations and more severe disease, although evidence from severe asthma cohort studies, such as the European Network For Understanding Mechanisms Of Severe Asthma (ENFUMOSA) and The Epidemiology and Natural History of Asthma: Outcomes and Treatment Regimens (TENOR) studies, show that allergens may play a less important role in severe compared to mild asthma. Occupational sensitisers and environmental factors (such as outdoor air pollutants and active or passive smoking) have been implicated in the increase in asthma morbidity since the early 1960s, and can be considered as inducers of the disease or triggers of exacerbations. Infections, mainly with respiratory viruses, but also with bacteria, such as Chlamydophila pneumoniae, contribute to severe asthma exacerbations, but their role in the development of severe persistent asthma remains unknown. Moreover, in individuals with severe asthma, a number of comorbid conditions are common, including psychopathology, hormonal dysfunction, obesity, rhinosinusitis, gastro-oesophageal reflux and sleep apnoea syndrome. All of these can lead to worsening of the asthma symptoms and a more severe phenotype of the disease. In order to improve the treatment of patients with severe asthma and to control the disease, all of these factors must be taken into consideration, and it is important to manage these conditions concurrently. ; -