Mg/Ca ratios in foraminiferal shells are widely used as a paleotemperature proxy, based on the positive relationship between magnesium incorporation into calcium carbonate and seawater temperature. Planktic foraminiferal Mg/Ca primarily reconstructs upper ocean temperatures, although non-thermal factors such as changes in seawater Mg/Ca, pH, salinity, and species-specific biological effects can influence temperature estimates. Benthic foraminiferal Mg/Ca is used to reconstruct bottom water temperatures but also requires consideration of non-thermal controls, particularly carbonate saturation. Both planktic and benthic Mg/Ca records can also be affected by preservation state and diagenetic processes. Overall, application of this proxy to generate robust Miocene ocean temperature reconstructions requires careful consideration of suitable Mg/Ca–temperature calibrations and consideration of non-thermal factors.