Early in its history, Earth differentiated into the core-mantle structure that defines its interior. Although this process had a profound effect on the planet, largely establishing the major and trace element compositions of the metallic core and silicate mantle, the planetary differentiation process itself remains largely a mystery. Important constraints on planetary differentiation are provided by experimental investigations of metal-silicate reactions at high pressures and temperatures, simulating the thermodynamic conditions at which much of the differentiation is thought to have occurred. These studies have shown, for example, that characteristic P,T conditions for core-mantle equilibration are comparable to the liquidus at mid-mantle depths, and that these reactions produce metal compositions that are broadly consistent with the geophysical constraints on Earth's core. The consequences of core-mantle differentiation on the planetary budgets of several trace elements will also be discussed.