CAR-T cells represent a new anti-tumor immunotherapy which has shown its clinical efficacy in B-cell malignancies. The results of clinical trials carried out in this context have shown that certain immunological characteristics of patients before (at the time of apheresis) and after the administration of the treatment, or of the CAR-T cells themselves, are correlated with the response to the treatment or to its toxicity. However, to date, there are no recommendations on the immunological monitoring of patients treated in real life. The objectives of this workshop were to determine, based on data from the literature and the experience of the centers, the immunological analyses to be carried out in patients treated with CAR-T cells. The recommendations relate to the characterization of the patient's immune cells at the time of apheresis, the characterization of the injected CAR-T cells, as well as the monitoring of the CAR-T cells and other parameters of immune reconstitution in the patient after administration of the treatment. Harmonization of practices will allow clinical-biological correlation studies to be carried out in patients treated in real life with the aim of identifying factors predictive of response and toxicity. Such data could have a major medico-economic impact by making it possible to identify the patients who will optimally benefit from these expensive treatments.