Debate has surrounded the subject of B cell life span since it was first measured in mice in the early 1970s. In the 25 years which have passed since then, it has become increasingly apparent that the methods employed to measure rates of B cell turnover, such as [3H]-thymidine labelling, cell transfer or cell ablation, brought about significant disruptions to normal physiology which in themselves might have affected B cell turnover. More recently the use of bromodeoxyuridine has overcome many of these methodological difficulties and has allowed rates of B cell renewal to be measured within B cell subpopulations defined by multiparameter flow cytometry. Such studies have largely resolved the issue, concluding that about 85% of peripheral B cells are phenotypically mature and display first-order exponential kinetics defined by a half-life of 5-6 weeks, whilst the remainder are short-lived with a life span of several days. This review examines both traditional and recent methods and discusses the influence of age, self-tolerance and randomness in the overall shaping of a kinetically stable mature B cell population.