T-cell acute lymphoblastic leukemia or lymphoblastic lymphoma (T-ALL/LBL) is a rare hematologic malignancy most commonly affecting adolescent and young adult (AYA) males. Outcomes are dismal for patients who relapse so improvement in treatment is needed. Nelarabine, a pro-drug of the deoxyguanosine analogue ara-G, is uniquely toxic to T-lymphoblasts, compared to B-lymphoblasts and normal lymphocytes, and has been developed for the treatment of T-ALL/LBL. Based on phase I and II trials in children and adults, single-agent nelarabine is approved for treatment of patients with R/R T-ALL/LBL, with the major adverse effect being central and peripheral neurotoxicity. Since its approval in 2005, nelarabine has been studied in combination with other chemotherapy agents for relapsed disease and is also being studied as a component of initial treatment in pediatric and adult patients. Here, we review current data with nelarabine and present our approach to the use of nelarabine in the treatment of patients with T-ALL/LBL.