This chapter addresses the use of trace metals in biogenic carbonates as proxies for past ocean conditions with an emphasis on the biogeochemistry of biomineralization. It illustrates the utility of trace metal proxies for reconstructing ocean conditions in the past, but also to emphasize how little is known about the detailed mechanisms that underlie these proxies. The chapter discusses the comparative trace metal geochemistries of coccolithophores, foraminifera, and corals relative to inorganic calcite and aragonite respectively, in order to define the differing nature of biological selectivity. It also addresses the biomineralization process for each organism and the biochemical nature of the trace metal selectivity involved at each step of the biomineralization. In order to monitor the reaction of our climate system to the exponentially increasing input of greenhouse gases since the industrial revolution, people require information from the subannual, decadal, and century scale. Foraminifera are unicellular calcifying marine amoeba, taxonomically part of the Protista.