Chimeric antigen receptor T (CAR T) cell therapy has achieved remarkable efficacy for patients with hematological malignancies. However, in vitro viral vector-mediated production of CAR T cells is time-consuming and expensive and impairs T cell function. On one hand, an elaborate manufacturing process not only impairs the function of CAR T cells but also limits its usage in patients with rapidly progressing diseases. On the other hand, high costs are incompatible with broad clinical applications for sizable populations. In vivo production of CAR T cells is a novel approach that can avoid complicated production processes and reduce costs through mass production. Additionally, in vivo production of CAR T cells does not damage the function of T cells compared with in vitro production. Early studies have demonstrated promising antitumor activity of in vivo CAR T cell therapy in preclinical models of hematological malignancies. In this review, we describe the latest developments of in vivo CAR T cell therapy and discuss its potential challenges for clinical application.