Published by the American Geophysical Union as part of the Geophysical Monograph Series, Volume 131. Scientists generally acknowledge that the reconstruction of past environments requires the use of multiple indicators or proxies. Biological remains, which figure prominently in many studies, offer different values for different purposes. Whereas scientists frequently use and well understand some biological proxies, for example, the foraminifers in marine environments and pollen grains in terrestrial settings, other biological proxies are underutilised. In this book, we focus on one such indicator of Quaternary paleoenvironmental phenomena: the ostracods or, as they are sometimes known, ostracodes. Ostracods are small aquatic crustaceans that secrete shells or valves made of calcite, which are often preserved in Quaternary sediments. In addition to providing a complementary line of evidence in multi-proxy investigations, ostracods have a number of specific advantages as biological proxies. First, they are sensitive to a range of ecological variables, such as habitat type and water composition. Second, their shells can be used in geochemical and isotope analyses and, because shell secretion is a rapid process, the results provide a "snapshot" of water conditions. Third, ostracods are often preserved in environments where other important biological proxies are not; in alkaline environments, for example, ostracods are well preserved, yet diatoms tend to be dissolved. Thus, we seek to encourage Quaternary scientists working both in marine and non-marine environments to make more routine use of ostracods in their work.