Publisher Summary This chapter briefly outlines the development of allergens focusing on identification of allergy and its mechanisms, on the relevance of allergen in the pathogenesis of asthma, and on increased prevalence of the condition(s). Allergy, which is type I IgE-mediated sensitization and sequelae to predominantly inhaled allergens, has become increasingly recognized as important in the pathogenesis of asthma. The disease was given the medical name Catarrhus aestivus (summer cold) but was recognized in the lay literature as hay fever, a label which persists to this day. This disease was felt to be limited to the middle and upper classes, and a major prevailing hypothesis was that it was due to the effects of summer heat and/or sunshine. The recognition of allergens as potential causes of asthma parallels the recognition of allergy. The identification of allergen-induced late asthmatic responses, allergen induced airway hyperresponsiveness, and allergen induced airway inflammation, that is features that define clinical asthma, has led to allergens becoming recognized as important position in the pathogenesis of asthma. Genetics play a role in determining the presence and severity of atopy, however, large changes in prevalence over a short period cannot be due to genetics and therefore must be due to one or more environmental factors. Chronic/recurrent allergen exposure may lead to persistent asthma. This provides a plausible basis to suggest that early prophylactically or actively anti-inflammatory therapeutic strategies should improve long-term outcomes in allergic asthma.