<p>Chimeric antigen receptor (CAR)-T cell therapy has transformed the outcomes of patients with haematological malignancies, yet its use is limited by labour-intensive manufacturing, constrained production capacity and variable clinical performance. In vivo CAR-T cell engineering, in which CAR-T cells are generated directly inside the patient’s body, seeks to overcome these challenges by eliminating the need for ex vivo cell processing and complex logistics, as well as improve clinical performance. Recent advances in virology, RNA medicines and nanotechnology have catalysed a radical overhaul of this approach, which uses targeted delivery systems such as lentiviral vectors and lipid nanoparticles to introduce CAR-encoding genetic material into endogenous T cells. Early clinical studies have shown efficient transduction, sustained CAR expression and initial signs of antitumour activity, establishing proof of concept. This Review explores the underlying technologies — including RNA delivered by lipid nanoparticles and engineered viral vectors — and discusses how they are being adapted to develop more broadly applicable, scalable, safe and effective CAR-T cell therapies. By removing the need for ex vivo manipulation and chemotherapeutic conditioning, this strategy could enable the wider application of CAR-T cell therapies not just to blood cancers but to autoimmune diseases for which ex vivo CAR-T cell therapies have shown strong promise, such as systemic lupus erythematosus.</p>