T-cell acute lymphoblastic leukemia (T-ALL) and T-cell lymphoblastic lymphoma (T-LBL) are considered the same disease, differing by the extent of bone marrow infiltration. According to recent gene expression profiling data, T-ALL and T-LBL can be separated by prediction analysis of microarrays showing an overexpression of MML1 in T-LBL and CD47 in T-ALL. Immunophenotypes of T-LBL and T-ALL are identical but differ in frequency, with a higher rate of cortical or mature immunophenotypes in T-LBL, which is probably related to the higher rate (> 90%) of mediastinal tumors. Treatment approaches in T-LBL changed from conventional non-Hodgkin lymphoma (NHL) protocols to intensive NHL protocols but recently to ALL-designed protocols. T-ALL remission rates are 90%, and overall survival (OS) has improved to 60%-70%. Mediastinal tumors resolve in most cases of T-ALL with chemotherapy only, whereas in T-LBL additional mediastinal irradiation seems to be beneficial. Strategies for stem cell transplantation (SCT) in T-LBL and T-ALL differ. Autologous SCT in complete remission (CR) in T-LBL gives a 70% survival rate, which is similar to chemotherapy alone. In T-ALL, the subtypes of early and mature T-ALL have a poor outcome with chemotherapy alone (< 30%) and might profit from an allogeneic transplantation in first CR (OS > 50%). There seems to be no need for transplantation in thymic T-ALL in first CR. Prognostic factors are published for T-ALL but not for T-LBL. MRD may guide further treatment strategies in T-ALL and probably also in T-LBL as indications for a SCT or for the evaluation of novel, particularly T-cell-specific, drugs.