Abstract CAR-T cell expansion and persistence are critical parameters for therapeutic efficacy and toxicity (Locke et al. 2020). However, CAR-T cells are patient-specific ‘living drugs’ with an unpredictable ability to expand in vivo. Thus, close postinfusion monitoring should be a major prerequisite to better manage this therapy. Critical parameters include CAR-T cell expansion kinetics and phenotype immune reconstitution and serum biomarkers (Fig. 35.1; Kalos et al. 2011; Hu and Huang 2020). Additionally, prospective collection and storage of patient specimens should be planned for future hypothesis-driven studies at specialized research centres. To date, despite the rapid expansion of CAR-T cell therapy, no standard recommendations exist for CAR monitoring, and harmonization of efforts across multiple centres is urgently needed.