The shell chemistry of ostracodes is a useful indicator of past environmental conditions especially when the chemistry data are considered along with other proxy data. The complexities involved with the chemical and isotopic changes accompanying hydrologic change, solute evolution, and the autoecology of ostracodes all point to the need to exercise caution when interpreting shell chemistry. Nevertheless, the stable-isotope values and cation ratios (e.g., Mg/Ca, Sr/Ca) as well as the species assemblage of ostracodes can provide powerful tools for the reconstruction of paleoclimate and paleohydrology. In particular, the changes in Mg/Ca and Sr/Ca of well-calcified ostracodes shells record the qualitative changes in solute composition, and when the dissolved Mg/Ca remains relatively constant, the Mg/Ca in the ostracode shell is proportional to water temperature.