Chromosomal mutations in human populations are of two forms: those which arise in germ cells, or their precursors, and result in constitutional chromosomal anomalies in offspring—and contribute to a substantial proportion of congenital abnormality and heritable diseases in man; and those that arise in somatic cells and which may result in cell killing, loss or diminution of cell function, or contribute to the processes involved in neoplastic transformation. The incidence of somatic chromosome mutations (aberrations) can be readily ascertained from studies on peripheral blood lymphocytes. Data on their prevalence in cells from “normal” populations are presented. The incidence of aberrations is increased in people exposed to mutagens so that changes in aberration frequency provide a means for monitoring population exposure. This is illustrated by studies on populations exposed to ionising radiations and to various chemical mutagens including cigarette smoke. An alternative approach to the cytogenetical assessment of chromosome damage is described, in which the incidence of lymphocytes resistant to cell killing by 8-azaguanine is utilised. The frequencies of such resistant cells are shown to increase with increasing age of the individual, and it is further demonstrated that their incidence is increased, in a dose-dependent fashion, when cells are exposed in vitro to X-rays or chemical mutagens.