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IL-25, also named IL-17E, is a distinct member of the IL-17 cytokine family, which can promote and augment T helper type 2 (Th2) responses locally or systemically. Growing evidence from experimental and clinical studies indicates that the expression of IL-25 and its cognate receptor, IL-17RB/RA, is markedly upregulated in asthmatic conditions. It has also been found that IL-25 induces not only typical eosinophilic inflammation and airway hyperresponsiveness (AHR), but also airway remodelling, manifested by goblet cell hyperplasia, subepithelial collagen deposition and angiogenesis. This review will focus on the discovery, cellular origins and targets of IL-25, and try to update current animal and human studies elucidating the roles of IL-25 in asthma. We conclude that although IL-25 is a pleiotropic cytokine, it may only play its dominant role in a certain specific asthmatic endotype, named ‘IL-25 high’ phenotype. Thus, targeting IL-25 or its receptor might selectively benefit some subgroups with asthma. Furthermore, the major IL-25 producing as well as responsive cells in the changeable milieu of asthma should be assessed in the future.