Chimeric antigen receptor (CAR) T cell therapy has had remarkable success targeting B cell leukemias in human patients, but unexpected toxicities and failures in other disease demonstrate the need for more predictive pre-clinical animal models than the murine ones currently used. Dogs develop spontaneous malignancies similar to humans in their tissues of origin, gene expression profiles, treatments, and disease courses, and have long been used as models for immunotherapy. I hypothesize that the development of CAR T cell therapy for dogs with spontaneous disease and that the treatment of these canine patients will recapitulate the observations found in human patients, and provide new insights into the safety and efficacy of this breakthrough therapy. To achieve this, I first established methods for growing primary canine T cells from healthy and disease-bearing donors to clinically relevant scale, developed RNA electroporation protocols to transiently express a CAR targeting the canine tumor-associated antigen CD20, demonstrated its function in vitro, and treated a relapsed canine B cell lymphoma patient with autologous CAR T cells as a proof of feasibility. I then developed methods to permanently express a second-generation cCD20-8-28-ζ CAR in canine T cells using lentiviral transduction, showed in vitro antigen-specific function and proliferation of CAR T cells, and treated four canine B cell lymphoma patients with CAR T cells. Based on my observations from those patients, I made iterative improvements to the T cell culture system and CAR construct, and treated a canine B cell lymphoma patient with cCD20-8-BB-ζ CAR T cells, whose tumor cells lost target antigen expression to avoid immune pressure. These results prove that it is not only possible to generate canine CAR T cell therapy, but that it recapitulates observations found until now only in human patients. In addition, novel findings regarding the recovery of T cells during ex vivo culture and the host immune response to the CAR demonstrate that this model can already inform human medicine.