Acute lymphoblastic leukemia (ALL) is the most common pediatric malignancy. Although outcomes for children with ALL have improved dramatically over the last 50 years, ALL remains the leading cause of childhood cancer death. In addition, high-risk patient subsets can be identified with significantly inferior survival. In the current era of therapies directed at specific molecular targets, the use of conventional randomized Phase III trials to show benefit from a new treatment regimen may not be feasible when these biologically defined subsets are small. This review presents the traditional approaches to designing trials for children with ALL, as well as innovative approaches attempting to study the benefit of new treatments as reliably as possible for patient subsets with distinctive biological characteristics.