Mastocytosis is a heterogeneous disease with an increased number and activation of mast cells. Subtypes range from benign to rare aggressive forms, and the disease may affect people of all ages. The pathogenesis involves mutations in the KIT gene in both children and adult patients. Estimated prevalence is one per 10,000, but the disease is very likely underdiagnosed. The diagnosis may be challenging and patients may present to several medical specialties. This article presents an overview of clinical signs and symptoms as well as a diagnostic algorithm and treatment options of mastocytosis.