Acute leukemia is a clonal expansion of tumoral cells in bone marrow, blood or other tissues. The acute leukemias are classified as myeloid or lymphoid based on the lineage of the blast cells. Over the past three decades, remarkable advances have been made in the classification and treatment of acute leukemias. In the last years, the research into the molecular pathogenesis of acute leukemia has progressed. The knowledge of chromosomal translocations breakpoints and possible candidate oncogenes and tumor suppressor genes has allowed the integration of all these events into multistep cascades that impact specific signal transduction pathways and lead to leukemic transformation.