This chapter focuses on the following topics related to the incidence of leukemia among children in the United States: (1) the relative frequencies of the leukemia subtypes that occur among children; (2) variation in the incidence of the specific types of leukemia by age; (3) differences in incidence between males and females; (4) differences in incidence between white andblack children; and (5) variation in leukemia incidence over time. In terms of survival for children with leukemia, the chapter focuses on three primary topics: (1)comparison of survival rates for children with ALL and AML; (2) the impact of age at diagnosis on survival; and (3) the remarkable improvements in survival rates for children with ALL during the past 20years. Before discussing topics related to childhood leukemia incidence and outcome, it is necessary to describe the specific diagnoses that are included among the Diagnostic Groups for leukemia of the International Classification of Childhood Cancer (ICCC). Table I.1 illustrates that acute lymphoblastic leukemia (ALL) accounted for approximately 99% of casesamong the lymphoid leukemia (Ia) diagnostic group, so that this ICCC diagnostic group is essentially synonymous with ALL. The “acute non-lymphocytic leukemia” diagnostic category Ib is henceforth referred to as the acute myeloid leukemia(AML) category since this is the preferred terminology [3], and it encompasses the various subtypes of AML that occur in children. The other three ICCC diagnostic categories combined accounted for only 6-7% of total leukemia cases in children. The chronic myeloid leukemias diagnostic group(Ic) included approximately 3% of leukemia cases occurring in the younger than 20years of age group during the period from1990-95, while the “other specified leukemia” diagnostic group (Id) included fewer than 1% of the leukemia cases