The role of iron in regulating the flux of carbon through the surface layer of the ocean has become increasingly apparent during the past 15 years. Before that time, the analytical challenges of measuring trace (parts per trillion) iron concentrations from iron ships using gear suspended on an iron wire precluded oceanographers from making accurate measurements. Laboratory experiments were invariably conducted with samples that were seriously contaminated with elevated iron concentrations. We now recognize, through greatly improved methodologies, that iron is a key regulator of phytoplankton primary production throughout the ocean. Small changes in iron concentration may produce large variations in the export of particulate organic carbon from the ocean's sunlit surface layer into deep-sea sediments. These variations in carbon export may occur over glacial/interglacial cycles at a scale sufficient to influence the flux of carbon dioxide from the atmosphere to the ocean. Such processes have been hypothesized to be an important driver of the changes in atmospheric carbon dioxide concentration that have been recorded in ice cores over the past 400,000 years.