IL‐25, also known as IL‐17E, is a member of the IL‐17 cytokine family mostly produced by epithelial cells and innate immune cells. After binding to the IL‐17RB/IL‐17RA complex, IL‐25 induces downstream signaling responses in epithelial cells and type 2 lymphocytes, which initiates, propagates, and sustains type 2 immunity. The function of IL‐25 in allergic diseases such as asthma has been well established, and now also is extended to diseases such as inflammatory bowel disease and cancer. This review summarizes the literature on IL‐25 and discusses the unsolved questions. Our knowledge on IL‐25 will pave the pathway for targeting this cytokine in inflammatory diseases.