We propose that the aryl hydrocarbon receptor (AhR), a unique chemical sensor, is critical in controlling mast cell differentiation, growth, and function in vitro and in vivo. In antigen-stimulated mast cells, exposure to AhR ligands resulted in a calcium- and reactive oxygen species (ROS)-dependent increase of reversible oxidation in and reduced activity of SHP-2 phosphatase, leading to enhanced mast cell signaling, degranulation, and mediator and cytokine release, as well as the in vivo anaphylactic response. Surprisingly, significant mast cell deficiency was noted in AhR-null mice due to defective calcium signaling and mitochondrial function, concomitant with reduced expression of c-kit and cytosolic STAT proteins, as well as enhanced intracellular ROS and apoptosis. Consequently, AhR-null mast cells responded poorly to stimulation, demonstrating a critical role of AhR signaling in maintaining mast cell homeostasis.