Mastocytosis is a heterogeneous group of disorders defined by abnormal growth and accumulation of clonal mast cells. CM is the predominant variant in childhood, whereas the majority of adult patients present with systemic mastocytosis. In over 80% of the latter patients, the KIT mutation D816V is detectable. Whereas childhood mastocytosis often resolves spontaneously before adolescence and has a good prognosis, in most adult patients mastocytosis is a persistent systemic disorder. The course and prognosis in these patients are variable; in indolent variants, the clinical course remains stable over years or even decades. In advanced mastocytosis, the prognosis is less favorable despite novel drugs counteracting growth of neoplastic mast cells. Although WHO criteria are clearly separating prognostic subsets of mastocytosis, little is known about specific prognostic factors predicting the outcome in individual patients. In this article, we review current concepts in mastocytosis, including prognostic parameters for indolent and advanced systemic mastocytosis.