Mast cells can function as effector and immunoregulatory cells in immunoglobulin E–associated allergic disorders, as well as in certain innate and adaptive immune responses. This review focuses on exciting new developments in the field of mast cell biology published in the past year. We highlight advances in the understanding of FcɛRI-mediated signaling and mast cell–activation events, as well as in the use of genetic models to study mast cell function in vivo. Finally, we discuss newly identified functions for mast cells or individual mast cell products, such as proteases and interleukin 10, in host defense, cardiovascular disease and tumor biology and in settings in which mast cells have anti-inflammatory or immunosuppressive functions.