The introduction of cranial radiotherapy (CRT) has provided efficient control of overt or subclinical meningeosis in acute leukemia. Especially due to the long-term toxicity of CRT, reduction or elimination of radiotherapy appeared mandatory after cure rates of more than 70% had been achieved in acute lymphoblastic leukemia (ALL). Several large clinical trials of the Berlin-Frankfurt-Münster (BFM) Study Group with more than 3500 patients since 1981 have demonstrated that intensive systemic and intrathecal chemotherapy without or with limited CRT can efficiently prevent central nervous system (CNS) relapses in a large percentage of patients. However, only in low-risk patients prophylactic radiotherapy can be completely and safely replaced by conventional doses of methotrexate. In addition, reduction of chemotherapy in low-risk ALL increased the rate of relapses with CNS involvement. Thus, only a combination of multidrug induction, high-dose methotrexate (HD-MTX) consolidation, and reintensification allowed safe elimination of CRT in low-risk ALL. This approach combined with CRT with 12Gy and 18 Gy in medium and high risk ALL, respectively, reduced the incidence of relapses with CNS involvement to less than 5% (trial ALL-BFM 86). Patients with inadequate response to therapy, or with T-cell ALL, or with overt CNS disease are at particularly high risk for relapse with CNS involvement, and require more systemic and intrathecal chemotherapy combined with cranial irradiation. In B-cell ALL, short intensive chemotherapy pulses including HD-MTX could completely replace radiotherapy. In AML, post-consolidation CRT appears to be advantageous with regard to control of extramedullary as well as systemic relapses.