This article summarizes the tremendous progress currently achieved in understanding the molecular basis of the pediatric acute leukemias. The article is organized from the perspective of the most frequently encountered pediatric acute leukemia genetic abnormalities in a molecular diagnostics laboratory setting. For each specific entity, the basic molecular biology, putative mechanisms of leukemogenesis, detection methods, and clinical significance are reviewed. Emphasis is placed on discussing the fusion genes generated from common nonrandom chromosomal translocations in B-lineage acute lymphoblastic leukemia (ALL), although brief summaries of T-lineage and myeloid leukemia, as well as the use of the antigen receptor gene rearrangement for residual disease monitoring in acute lympocytic leukemia are also presented. Finally, an overview of emerging technologies of potential importance in the laboratory diagnosis and evaluation of the pediatric acute leukemias is provided.