How can we know the surface ocean temperature near the equator during the last ice age? This is the fundamental quandary of paleoceanography: how to reconstruct environmental variables that can no longer be measured directly. Proxies are chemical, physical or biological cues that correlate with the parameter of interest (e.g., temperature, pH, nutrient status) in a predictable fashion, and that are well preserved in the geological record over thousands or even millions of years. New proxies are constantly developed, and existing proxies are refined and improved. Researchers in our department have a strong track record in the development, calibration and validation of new and existing proxies. These proxies help us understand how the Earth's surface environment has changed through time, and what processes and feedbacks have driven this environmental change.