A large number of certified and other reference samples are available for use in analytical geochemistry. Certified materials are preferred, but of much more limited availability than other reference samples for most geochemical applications. The availability of rock, sediment, soil, water, and plant reference samples is outlined; ore and mineral separate reference samples are not included in the discussion. The preparation of these materials, including the establishment of certified or recommended concentrations, is then reviewed. It is shown that comparable quality can be achieved for both certified and recommended concentrations, though it has not always been achieved in the past. Finally, the most appropriate ways to use reference samples in quality control and instrumental calibration are discussed. ?? 1992.