In mast cell (MC) disorders (mastocytosis), clinical symptoms are caused by the release of chemical mediators from MCs, the pathologic infiltration of neoplastic MCs in tissues, or both. Cutaneous mastocytosis is a benign disease in which MC infiltration is confined to the skin. In pediatric cases cutaneous mastocytosis might regress spontaneously. Systemic mastocytosis (SM) is more frequently diagnosed in adults and is a persistent (clonal) disease of bone marrow–derived myelomastocytic progenitors. The somatic c-kit mutation D816V is found in the majority of such patients. The natural clinical course in SM is variable. Whereas most patients remain at the indolent stage for many years, some have aggressive SM (ASM) at diagnosis. Other patients have an associated clonal hematologic non–MC lineage disease (AHNMD). MC leukemia (MCL) is a rare disease variant characterized by circulating MCs and fatal disease progression. The diagnoses of ASM, SM-AHNMD, and MCL might be confused with a variety of endocrinologic, vascular, or immunologic disorders. It is therefore of particular importance to be aware of the possibility of an underlying (malignant) MC disease in patients with unexplained vascular instability, unexplained (anaphylactoid) shock, idiopathic flushing, diarrhea, headache, and other symptoms that might be mediator related. An important diagnostic clue in such cases is an increased serum tryptase level. The current review provides an overview of mastocytosis and its subvariants and a practical guide that might help to delineate mastocytosis from unrelated systemic disorders.