The term systemic mastocytosis (SM) encompasses a group of hematopoietic malignancies characterized by excessive proliferation of neoplastic mast cells that accumulate in the bone marrow and visceral organs. Most patients with SM, particularly those who present with aggressive clinical courses, carry somatic mutations of the v-kit Hardy-Zuckerman 4 feline sarcoma viral oncogene homolog (KIT) gene. KIT mutations are considered central events in the pathogenesis of SM and serve as diagnostic markers and putative therapeutic targets. The heterogeneity in the clinical course of patients with SM and recent advances in the genetic and immunophenotypic characterization of neoplastic mast cells may help to improve current diagnostic, taxonomic, and therapeutic approaches in SM. Cancer 2011;. © 2011 American Cancer Society.