Humans have a remarkable ability to maintain relatively constant lymphocyte numbers across many decades, from puberty to old-age, despite a multitude of infectious and other challenges and a dramatic decline in thymic output. This phenomenon, lymphocyte homeostasis, is achieved by matching the production, death, and phenotype transition rates across a network of varied lymphocyte subpopulations. Understanding this process in humans depends on the ability to measure in vivo rates of lymphocyte production and loss. Such investigations have been greatly facilitated by the advent of stable isotope labeling approaches, which use the rate of incorporation of a tracer into cellular DNA as a marker of cell division. Two labeling approaches are commonly employed, one using deuterium-labeled glucose and the other using deuterium-labeled water, also known as heavy water ((2)H(2)O). Here we describe the application of these two labeling techniques for measurement of human in vivo lymphocyte kinetics through the four phases of investigation: labeling, -sampling, analysis, and interpretation.