During and following their accretion, Earth and other terrestrial planets underwent dramatic changes as they differentiated into their constituent reservoirs (core, mantle, crust, atmosphere). This session explores the petrological evidence for the existence and nature of these early reservoirs and their associated formation processes. The broad scope, spanning the deepest parts of planetary cores through to the outer wisps of exosphere in the early history of planets, is intended to help shape an inter-connected planetary-scale approach to understanding early planet evolution. Suggested topics of interest include, but are not limited to: the sampling of hypothetical primordial mantle by igneous processes on Earth; the geochemical fingerprints of core formation; records of the presence, or loss of, volatile elements in early atmospheres; and magmatic records of mantle heterogeneity from magma ocean fractionation in early Mars and early Moon history. We anticipate a focus on elemental and isotopic geochemical approaches; supporting modelling, experimental and other approaches also welcome.