Trace elements and isotopes underlie many of the proxies used to reconstruct past ocean conditions. These proxies, recorded in diverse archives, are used to reconstruct seawater properties such as temperature, pH, and oxygen, or oceanic processes such as circulation, nutrient uptake, and biological productivity. Proxy calibration and validation requires a combination of ocean sediment core-top measurements, sediment trap studies, and laboratory- or field-based observations. New measurements of proxies in the modern ocean are rapidly illuminating the scope and limitations of each proxy while also helping to identify and evaluate new geochemical proxies that are based on trace elements and their isotopes.