The measurement of the delta13C-values of biological tissues, including remnant human collagen from archaeological sites, is a new technique used to obtain information on the diets of the population studied. This paper reviews several projects that we have undertaken to determine the accuracy of this technique as applied to prehistoric human populations, particulary those inhabiting coastal areas. We conclude that the relative percentages of marine and terrestrial foods consumed by an individual can be measured to an accuracy of approximately 10per thousand at one standard deviation.