The greatest advance in precision medicine for asthma has been the development of specific anti-eosinophil treatments, which target mediators involved in eosinophil inflammation in the airways. Asthma is usually characterized by chronic inflammation of the airways, with an infiltration of activated eosinophils orchestrated by dendritic cells that interact with T helper 2 (Th2) cells or from nonallergic mechanisms that lead to activation of innate type 2 lymphocytes (ILC2). Th2 and ILC2 cells express the transcription factor GATA3, which results in the synthesis of IL-5, which is required for eosinophil differentiation, proliferation, and survival. Anti-IL-5 blocking antibodies, which were remarkably effective in animal models of allergic asthma, are therefore a logical approach to the treatment of inflammation in asthma. The first human study of an anti-IL-5 antibody (mepolizumab) showed a profound and prolonged fall in circulating eosinophils and a marked reduction in sputum eosinophils after allergen challenge in patients with mild asthma. Yet there was no reduction in the early or late response to inhaled allergen or in airway hyperresponsiveness, which is probably explained by the fact that there was no effect on mast cell activation, which is thought to mediate many of the day-to-day symptoms of asthma. In unselected patients with asthma whose asthma was not controlled by inhaled corticosteroids (ICS), there was no clinical improvement despite a marked reduction in eosinophils in blood and sputum. Since then, several clinical studies have been performed in selected patients with refractory asthma as well as persistent eosinophils in sputum and a history of frequent exacerbations. These studies consistently show a reduction, but not elimination, of exacerbations, with little effect on symptoms, lung function, or quality of life. Thus anti-IL-5 therapy would not be a sufficient treatment alone, probably because it does not affect the mast cell activation that produces most asthma symptoms. It seems that patient selection is very important, but patients with refractory asthma that is not controlled by maximal inhaled therapy or oral corticosteroids and who have increased eosinophils in sputum and a history of frequent exacerbations are relatively uncommon and usually explained by poor adherence to ICS. Another anti-IL-5 antibody, reslizumab, and an anti-IL-5 receptor antibody (IL-5Rα), benralizumab, have clinical effects similar to those of mepolizumab., Anti-IL-5 treatments have now been approved for use in refractory eosinophilic asthma, although in the United Kingdom, the National Institute for Health and Care Excellence has not approved mepolizumab on cost-effectiveness grounds.