Acute lymphoblastic leukemia (ALL) accounts for 25% of childhood cancer. Clinical and biological parameters define prognostic categories and therapeutic approaches. For the majority of children with B-progenitor ALL, age (1-9 yrs) and white blood count (WBC50,000 and age>9 yrs. define eligibility for high-risk protocols. Infants (50,000; other criteria may include B-progenitor ALL with WBC>100,000,philadelphia chromosome positive ALL, residual marrow disease at day 7, or male gender, For the 5% of children with CNS leukemia at diagnosis (defined as CSF with 5 or greater WBC/uL and positive cytology), Crl is administered at does of 18 to 24 Gy. Recent series show a CNS relapse rate approximating 5% Reinduction chemotherapy, IT therapy, and subsequent craniospinal irradiation ( typically 24, Gy Crl, 15 Gy spine) achieve durable secondary disease control in greater than 60% of cases with isolated CNS relapse. High-dose systemic MTX has virtually eliminated testicular relapse. CNS toxicities (including leukoencephalopathy) relate to systemic and IT MTX as well as Crl. Late neuropsychologicral data following Crl at 18 Gy show little intellectual deficit compared with children receiving MTX- based preventive therapy alone. Neuroendocrine dysfunction is apparent in long-term survivors following Crl at 24 Gy. The relative efficacy and toxicities of Crl versus more intensive MTX-based regimens are yet controversial. Overall disease-free survival in ALL approximates 70% in contemporary series.