Mast cells are important effectors in innate and adaptive immune responses. They contain numerous secretory granules filled with inflammatory mediators in their cytoplasm. Exocytosis of granular content does not take place until the cell receives an appropriate stimulus such as the aggregation of IgE antibody bound to high‐affinity IgE receptors by specific antigen. This process is therefore referred to as regulated exocytosis. A characteristic of mast cell exocytosis is that it does not involve release of a few individual granules, but rather, a large fraction of the granular content is released due to compound exocytosis, implicating the occurrence of granule‐to‐granule and granule‐to‐plasma membrane fusion. This unit describes assays that measure the release of granular content from mast cells. They include in vitro colorimetric‐, radiolabel‐, or antibody detection–based assays for substances stored in the granules. Given that animal models for basophil and mast cell activation are being used with increasing frequency, the unit also includes protocols to measure exocytosis of mast cell granule content in vivo.