It is known that patients with mastocytosis have an increased risk of anaphylaxis. This also appears to be the case with patients with evidence of a clonal mast cell disorder resulting in the monoclonal mast cell activation syndrome (MMAS) who do not express the full mastocytosis phenotype. Most patients with mastocytosis are recognized by their characteristic skin lesions. An increased level of baseline serum mast cell tryptase is also an indicator for a possible clonal mast cell disorder including mastocytosis. Other markers for mast cell clonality and for mastocytosis include abnormal immunostaining of mast cells with CD25 and CD2, clustering of mast cells in tissues, abnormal mast cell morphology, and the presence of a mutation in the proto-oncogene c-kit encoding for the mast cell growth receptor KIT. As recognition depends on an understanding of mastocytosis, and this disease should be considered in patients with recurrent anaphylaxis, we describe the features of mast cell clonality, MMAS and mastocytosis, and review recent findings.