Aqueous fluids and melts (silicate, carbonate/carbonatites or metallic melts) are major vectors of mass and heat transfer in planetary interiors and play an important role in a variety of geochemical and geodynamical processes that shape the internal and surficial evolution of planetary systems. These processes occur over a broad range of pressure and temperatures that span from shallow crustal conditions (e.g., magma chamber processes, ore deposit formation) to the deep core-mantle boundaries (e.g., planetary differentiation), and they are strongly influenced by the physical, chemical and structural properties of aqueous fluids and melts at relevant pressure and temperature conditions. Over the last decade, major progress has been made in the determination of the properties of aqueous fluids and melts thanks to the development of advanced experimental and computational methods. The goal of this special issue is to highlight some of these advancements and to discuss new views and open questions related to mass transfer and melting in planetary interiors, from crustal conditions to core-mantle boundaries and from the atomic to the planetary scales.