The incidence of allergic diseases has dramatically increased in recent decades, especially in urban and industrialized areas. It is important socially as well as medically to establish more useful strategies to overcome allergic disorders. Bronchial asthma is a complex disease characterized by airway inflammation involving a Th2-cytokine, interleukin (IL)-13. A substantial body of evidence has accumulated pointing to the pivotal role of IL-13 in the pathogenesis of bronchial asthma, based on mainly analyses of mouse models. In addition to such analyses, the high expression of IL-13 in lesions and genetic association of several genes coding IL-13 signaling molecules with bronchial asthma have raised the possibility that IL-13 plays a pivotal role in the onset or exacerbation of human bronchial asthma. Therefore, IL-13 and its signal pathway are thought to be promising targets to develop a therapeutic agent for bronchial asthma. In this article, we describe how IL-13 is involved in the pathogenesis of bronchial asthma and then how therapeutic agents to block IL-13 signals are developed for bronchial asthma.