Acute lymphoblastic leukemia accounts for 80% of leukemia in children. The exact cause is unknown, but some genetic, immunologic, viral, and environmental factors have been implicated. Symptoms at the time of diagnosis frequently include fever, bleeding, fatigue, and irritability. Initial white blood cell count and patient age at diagnosis are the most reliable indicators of prognosis. Acute lymphoblastic leukemia is a heterogenous disease. Lymphoblast morphology, immunologic markers, enzyme abnormalities, cytogenetic findings, and staining characteristics in conjunction with clinical characteristics allow classification into risk groups. Appropriate therapy for each risk group is based on these parameters. Combination chemotherapy administered alone or with additional chemotherapy or radiotherapy to sanctuary sites is the principal modality for treatment of ALL. Optimal therapy for relapse has not yet been determined, but for patients with appropriate donors, allogeneic bone marrow transplant is promising. Common complications of chemotherapy include tumor lysis syndrome, myelosuppression, and other problems such as gastrointestinal toxicity, neurotoxicity and cardiac toxicity. Significant late effects of chemotherapy include neurological impairment ranging from learning problems to leukoencephalopathy and a possible increased risk of second malignancy. Complete remission is achieved in 95% of children with acute lymphoblastic leukemia, and more than 55% will continue to be in complete remission at five years. Optimal CNS prophylaxis, effective treatment of relapse, and adjustment of therapy to minimize acute and late adverse effects are a continuing challenge. With improved understanding of biologic factors, and development of more specific therapy for each subgroup, children with acute lymphoblastic leukemia should enjoy a better long term outcome.