The different lymphocyte populations of the immune system are maintained at fairly constant numbers throughout life. The importance of this lifelong maintenance is illustrated by clinical conditions of lymphopenia, such as human immunodeficiency virus infection, severe combined immune deficiency, or hematopoietic stem cell transplantation, when opportunistic infections can cause considerable morbidity and mortality. Maintenance of lymphocyte populations is a dynamic process in which cell production and loss are balanced. Immunologists have adopted a variety of approaches to address questions related to these lymphocyte dynamics. Yet distilling the desired information from experimental data has remained a challenge, even with the aid of mathematical models. This thesis integrates experimental approaches and mathematical modeling to gain insights into the biology of human lymphocyte populations. The key aims of this research were 1) to improve the interpretation of experimental data in order to provide the most reliable information about lymphocyte dynamics, which should help explain discrepancies in the literature and reach consensus in the field); and 2) to gain insights into the lifelong maintenance of the peripheral lymphocyte pools in healthy humans. We found that by using a multi-exponential model – instead of a commonly used single-exponential model – for the interpretation of stable isotope-labeling data we can obtain reliable estimates of the average turnover rate of kinetically heterogeneous cell populations, i.e., populations that consist of multiple subpopulations with distinct turnover rates, such as the memory T-cell pool. These and other methodological insights reported in this thesis provide a major step toward consensus about lymphocyte turnover. Moreover, by a combination of methods, primarily stable isotope-labeling techniques and T-cell receptor excision circle analyses, we gained important biological insights. First, in humans the major source of new naive T cells is fundamentally different from that in mice: the majority of new T cells are produced by peripheral division, while thymic output has a very limited role throughout adulthood. Thymic output in humans declines from an estimated 16 million to 1 million cells per day, thereby contributing to the total production of naive T cells for only 10% in young adults and for as little as 1% in the elderly. Second, maintenance of several T-cell and B-cell populations during healthy aging did not require substantial alterations in the turnover rates, and we found no signs of homeostatic compensation (by increased cell division or survival) in response to declining thymic output and naive T-cell numbers. The results described in this thesis directly bear upon our knowledge of immune function in healthy individuals, and, together with the methodological insights form the basis for future research into aging and conditions in which lymphocyte dynamics are disturbed.