Mastocytosis is a disorder characterized by presence of excessive numbers of mast cells in skin, bone marrow and internal organs. It can affect both children and adults, males and females and individuals from all ethnic backgrounds, although precise demographic information about the affected populations is not available as it is a rare disorder. Mastocytosis in children is generally limited to the skin and follows a self limited course, while it is a disorder of the hematopoietic stem cell associated with somatic mutations of the c-kit gene in most patients with adult-onset of disease. There is no known curative therapy for most patients with systemic mastocytosis. Recent research studies identified several subtypes of disease with distinct clinical and pathologic features, however, a precise understanding of the incidence as well as molecular pathology of different disease subtypes is lacking. This study aims to examine molecular and cellular pathological aspects of disease in patients with mastocytosis and correlate findings with clinical presentation and prognosis. Patients will undergo a routine history and physical examination, and diagnostic tests will be ordered as dictated by each patient's clinical presentation. Blood and bone marrow will be obtained for diagnostic and research purposes. Genetic analysis of the c-kit gene regulating mast cell growth and differentiation will be performed. It is hoped that findings obtained from this study will help to design novel therapies for mastocytosis and other disorders in which mast cells play a critical role.