This article updates current knowledge about epidemiology, prognosis, and risk factors for major complications in mastocytosis. A prevalence of mastocytosis of 1 in 10000 inhabitants has been reported, but underdiagnosis is assumed. The prognosis for cutaneous and indolent systemic mastocytosis is excellent. For more advanced forms of disease, prognostic parameters have been identified. A high extent of skin involvement, increased basal serum tryptase values, and extensive blistering are risk factors for severe mast cell activation episodes in children, whereas these associations seem to be less strong or nonexistent for anaphylaxis and osteoporosis in adult patients with indolent systemic mastocytosis.