Mg/Ca palaeothermometry provides a means of constructing quantitative records of Cenozoic climate change. Its application is not limited to past temperature estimates, for when this proxy is coupled with oxygen isotope palaeothermometry, past variations in the oxygen iso-topic composition of seawater may also be estimated. In this way, the Cenozoic history of continental ice-sheet growth has been outlined. Further insights into the dynamics of the Cenozoic climate system have arisen from regional seawater salinity reconstructions, which shed light on changes in ocean circulation patterns. Finally, Mg/Ca palaeothermometry provides a means of testing the accuracy of climate models simulating past warm climates.