Isotope ratios provide important opportunities for understanding the physical and chemical processes that shape the modern Earth system and that have controlled the evolution of the Earth over its 4.55 billion year history. These measurements are made primarily on the Thermo Neptune (a multi-collector, sector field, plasma-source mass spectrometer). Using this instrumentation, researchers at CEMS now routinely measure isotope ratios for a diverse suite of elements, including Si, B, Fe, Zn, Cd, Sr, Nd, Hf and Pb.  These data are used to understand a wide variety of processes, including those controlling the composition of modern seawater, and those that shaped the formation of the earliest continental crust.