Constraining mantle composition and melting processes is crucial for our understanding of planetary dynamics and evolution. After decades of work, some fundamental questions regarding source composition, melt migration, storage, and evolution remain unanswered. In order to advance our understanding, we will bring together studies that combine the strengths of geochemistry, petrology, geology, geodynamics, geophysics, and/or mineral physics. We invite contributions that study melting of the asthenosphere or lithosphere in various tectonic environments (e.g., mid-ocean ridges, subduction zones, hotspots), as well as deep planetary melting (e.g., transition zone, magma ocean). Topics covered by this session include but are not limited to: (1) magmatic processes in the presence of volatiles and heterogeneous rock assemblages, (2) melt migration and melt-rock interaction, (3) magmatic processes in the early Earth, (4) plume vs. non-plume origin of intraplate volcanism, (5) evolution of melts in the lithosphere, or (6) extraterrestrial volcanism on rocky planets.