Mast cells and basophils share many characteristics, such as surface expression of the high affinity receptor of IgE, FcεRI, granule storage of histamine, which is released during their activation, and potentials to produce pro-/anti-inflammatory cytokines. These similar leukocytes, however, were found to have their own process of differentiation. Indeed, accumulating evidence suggests that these cells should play critical roles in type I allergy including anaphylaxis and in urticaria. Various inflammatory mediators derived from mast cells/basophils, such as histamine, platelet-activating factor, prostanoids, and leukotrienes, have been paid a particular attention to as the therapeutic targets for type I allergy and inflammatory diseases. Recent progress in the field of mast cell/basophil research has shed light on their physiological roles in bacterial infection, energy metabolism, and cutaneous/intestinal inflammation. This review makes a brief introduction of these recent studies, which are expected to provide novel therapeutic approaches for infectious and chronic inflammatory diseases.