Mast cells (MC) are regular constituents of soft tissue and occur with varying frequency in nearly every organ. They derive from monocytic cells occurring in the adhering mononuclear fraction of the peripheral blood. Their subsequent evolution into mature MCs is primed by a MC generating lymphokine released by sensitized T-cells on restimulation by the antigen. MC granules contain preformed heparin histamine and eosinophil chemotactic factors. Other factors such as leukotriene B4 can be produced by MCs following stimulation. This is the case during the initial phase of nonspecific inflammations, when MCs are stimulated by complement activation. In the immediate type hypersensitivity reaction giving rise to IgE, MC degranulation occurs independent from complement. In IgG and IgM mediated reactions, however, MC involvement is effected by complement consumption and C5 a generation. In delayed type hypersensitivity MCs increase locally. Their functional significance remains obscure. MC neoplasias are rare and generally confined to the dermis. Cutaneous mastocytoses are called benign mastocytoma when localized and urticaria pigmentosa when disseminated. Generalized mastocytosis involves extracutaneous tissue irrespective of skin involvement. Those associated with urticaria pigmentosa-like skin lesions, present at the onset of the disease, have a significantly higher survival rate than those lacking a primary skin involvement. The term urticaria pigmentosa should be reserved for cases of cutaneous mastocytosis without extracutaneous involvement. Cases of mastocytoses lacking primary skin lesions assume a malignant course and are additionally aggravated by high incidence of myeloproliferative disorders and MC leukemia. MC sarcoma is an extremely rare neoplasia of MCs which may also terminate as a MC leukemia.