Mastocytosis has been defined in the literature as an abnormal accumulation of tissue mast cells in one or more organ systems. Broadly separated into two categories - cutaneous mastocytosis (CM) and systemic mastocytosis (SM), the disease occurs in both children and adults. CM is a benign skin disease representing the majority of pediatric cases. It often resolves during puberty and is not associated with systemic involvement, but because only a small subgroup of people with CM display a KIT mutation, not much is known about the factors that contribute to the accumulation of mast cells in the skin. Alternatively, most adult patients are diagnosed with SM. Skin involvement, typically urticaria pigmentosa, is common in adult patients and can provide an important clue to accurate diagnosis. In all SM categories, the common histological marker is the clustering of mast cells in visceral organs.