This purpose of this study is to determine the efficacy and safety of tisagenlecleucel (also referred to as CTL019, or Kymriah™) in children, adolescents and young adults with relapsed/refractory B-cell non-Hodgkin lymphoma (NHL). The study is enrolling children and young adults up to age 25 with CD19-expressing mature B-cell NHL, including Burkitt leukemia/lymphoma, that has relapsed after one or more prior therapies or is primary refractory (not achieved a complete remission (CR) or partial response (PR) after the first line of therapy). This study involves the administration of the study drug, blood draws, assessment of lymphoma response, and chart reviews. After a process called leukapheresis, in which T cells (a type of white blood cell) are removed from the subject's blood and modified in order to target the lymphoma, they are returned back to the subject as the study drug. Reimbursement for travel may be provided. If you have questions or would like to learn more about the study, please contact the Cancer Immunotherapy Program at (215) 425-7193. The Principal Investigator is Dr. Shannon Maude.