Leukemias and lymphomas, the most common type of neoplasms in children, represent a heterogenous group of diseases that display a wide spectrum of clinical, morphologic, immunologic, cytogenetic, and molecular features. Modern treatment is curative for most children with lymphoid neoplasms; however, new approaches to therapy of acute nonlymphoblastic leukemias are necessary if current results are to be improved. This article focuses on the clinical and biologic characteristics, and the modalities of treatment of this group of diverse diseases.