Asthma is a chronic inflammatory disease of the airways characterized by variable airway obstruction. In some patients with severe disease there are frequent disease flares, and some individuals develop irreversible airway obstruction. The immune system has a predominant effect on many aspects of the disease. A large proportion of people with asthma have signs of type 2 immunity, rich in eosinophils, mast cells and basophils, and controlled by either type 2 helper T cells or type 2 innate lymphoid cells. Other patients have a more neutrophil-predominant disease, and some have little underlying immune dysfunction. Here we review the immunology of asthma by integrating data from mouse model studies with clinical intervention studies.