The research presented in this thesis is focused on the quantification of lymphocyte dynamics by in vivo deuterium labelling. To understand how lymphocytes are maintained in homeostasis and when the homeostatic balance is disrupted, it is essential to quantify their production and loss rates. Hereby, this thesis revealed three main observations. First, the turnover rate of antigen‐specific T‐cells against mouse cytomegalovirus (MCMV) is independent of the size of the specific memory T‐cell response and similar to that of bulk memory‐phenotype T‐cells. Second, the maintenance of memory T‐cells in the bone marrow (BM) is a very dynamic process, in which cells constantly recirculate and self‐renew. Last, lymphocyte production and loss rates do not seem to be homeostatically regulated in a density‐dependent manner during immune reconstitution following autologous hematopoietic stem cell transplantation (HSCT) in humans.