About 5% of asthmatics do not behave like ‘classical’ asthmatics and may not respond adequately to conventional therapy. The terms used to describe such non-responders include severe, refractory, near fatal, difficult and difficult to control asthma. Within the umbrella term of severe or refectory asthma, there are distinct sub-phenotypes including brittle asthma. Brittle asthma is rare and may occur in 0.05% of all asthmatics. Currently the diagnosis of brittle asthma is made on clinical grounds based on the variability of peak flow and uncertainty and unpredictability of sudden onset of disabling and severe symptoms despite maximal medical therapy with high dose inhaled corticosteroids, inhaled and nebulised bronchodilators and either maintenance or repeated courses of systemic corticosteroids. The role of genetics, environmental exposure and infection is the focus of ongoing research in the development of severe asthma. Atopy, female sex and psychosocial factors are recognised to be associated with brittle asthma. Other factors, investigated as possible initiating or contributing factors in brittle asthma include nutrient deficiency, reduced antioxidants activity and immunodeficiency with low IgG subclass levels. This review will highlight the related phenotypes, risk factors, mortality and morbidity, pathogenesis and management of patients with brittle asthma.