Anaphylaxis results from mast cell degranulation induced by allergen-specific IgE as well as various non-IgE-mediated mechanisms. It has been recently demonstrated that intrinsic abnormalities in mast cells, such as presence of activating D816V c-kit mutation, may influence susceptibility to anaphylaxis, especially in patients with “idiopathic” or hymenoptera-induced anaphylaxis. However, despite an improved understanding of the role of clonal mast cell disease in susceptibility to anaphylaxis, the basis of an apparent increase in susceptibility in the majority of patients remains poorly understood. In this chapter, we will review the potential mechanisms of mast cell activation as well as the range of symptoms and the differential diagnosis of patients suspected of having a disease caused by mast cell activation. In addition, we offer a global classification for disorders involving mast cells.