Although the survival rate for adolescents and young adults (AYA) with acute lymphoblastic leukemia (ALL) has steadily improved over the past several decades, it still lags behind that of younger children. This Review explores the reasons for this discrepancy and potential solutions, focusing on patients aged 15-22 years. Recent studies that compared the outcome of AYA patients with ALL treated on pediatric or adult clinical trials have shown substantially better outcomes for this patient population obtained with the pediatric trials. Excellent early results have been obtained for patients with ALL aged up to 40-60 years who were treated in adult study groups with pediatric-based regimens. Targeting biological and socio-political features unique to AYA ALL has reduced the 'AYA gap' and has provided the foundation for basic science and translational and clinical AYA initiatives that are charged with the task of discovering further methods to improve the outcome of AYA with ALL.