Mastocytosis comprises a heterogeneous group of disorders characterized by clonal, neoplastic proliferation of mast cells accumulating in one or multiple organs. In the majority of cases skin involvement is the first clinical manifestation of the disease. Clinical work-up consists of a combination of morphological, immunohistochemical, flow cytometric immunophenotyping and molecular examination. Cutaneous mastocytosis predominates in children, whereas systemic mastocytosis is the most common form of the disease in adults. Therefore, different diagnostic algorithms have to be applied in adult patients and children with suspected mastocytosis. This comprehensive review presents currently defined variants of the disease and recommendations to facilitate diagnostic work-up in children and adults with suspected mastocytosis in daily clinical practice.