Mast cells (MCs) are major effectors of IgE-mediated allergic reactions because of their unique peripheral location and their powerful capacity to release prestored and de novo-synthesized inflammatory mediators into the circulation upon activation. In view of the growing incidence of allergy worldwide, there is much interest in developing novel strategies to block or temper IgE-mediated MC activation and its pathological consequences. For these studies, standard assays to measure IgE-mediated MC degranulation and mediator release are required. Here, we present detailed procedures to assess in vitro and in vivo MC release of prestored as well as recently synthesized mediators following IgE-/antigen-mediated activation.