Ostracodes are microcrustaceans that live in much of Earth's aquatic realm. They make a bivalved shell (carapace) composed of calcite which is commonly preserved in the sediment record and has been increasingly used as a geochemical tracer of past environmental conditions. A series of calibration studies have been conducted to evaluate the elemental chemistry of benthic marine ostracode shells as a paleoenvironmental tracer. Research has focused on metal:calcium ratios (Mg/Ca, Sr/Ca, and Na/Ca) in the shells of two common genera: deep-sea genus Krithe and estuarine/shelf genus Loxoconcha. Results indicate phylogenetic and ontogenetic effects, and confirm a strong Mg/Ca-thermodependence. Thus, of the three ratios, Mg/Ca offers the most promise as a tracer of past water temperature. Diagenetic effects (dissolution) on Mg/Ca appear to be small. Initial Mg/Ca-temperature calibrations have been applied to late Quaternary sediment cores from the North Atlantic and Arctic Oceans and from eastern U.S. estuaries at time-scales ranging from Milankovitch to inter-annual. When possible, the Mg/Ca-based temperature records have been coupled with benthic oxygen isotopes. This coupled-proxy approach provides records of the oxygen isotopic composition of past ocean and estuarine waters, which in turn can be used to assess changes in global ice volume and changes in estuarine salinity/terrestrial runoff, respectively.