Asthma is characterized by chronic airway inflammation with intense eosinophil and lymphocyte infiltration, mucus hyperproduction, and airway hyperresponsiveness to a variety of stimuli. It is now generally accepted that antigen-specific Th2 cells and their cytokines orchestrate these pathognomonic features of asthma. On the other hand, Th17 cells and IL-23, a cytokine that preferentially expands Th17 cells, play a significant role in the development of chronic inflammatory diseases, including autoimmune diseases. Recently, we have shown that IL-23 and Th17 cells enhance not only neutrophilic airway inflammation but also Th2 cell-mediated eosinophilic airway inflammation in a murine asthma model. In this review, we will discuss the roles of IL-23 and Th17 cells in airway inflammation in asthma.