Severe equine asthma (EA) is the newly adopted term which defines the syndromes most recently known as recurrent airway obstruction (RAO; syn chronic obstructive pulmonary disease, Heaves) or summer pasture associated recurrent airway obstruction (SPARAO; syn summer pasture associated obstructive pulmonary disease). The adoption of this new nomenclature recognises the clinical phenotypic overlap amongst various non-infectious diseases of the equine lower airways; most notably between severe EA and mild to moderate EA (previously termed inflammatory airway disease). Compared with mild to moderate EA, there is a greater appreciation of the role of genetics in determining susceptibility to severe EA and a better understanding of aetiological triggers and disease pathogenesis. However, despite the relatively well defined clinical phenotype of severe EA, it is increasingly apparent that a significant degree of heterogeneity exists with respect to the susceptibility determinants and underlying immunological mechanisms which ultimately lead to clinical disease.