Partial melting of the continental crust has long been of interest to petrologists as a small-scale phenomenon. Mineral assemblages in the cores of old, eroded mountain chains that formed where continents collided show that the continental crust was buried deeply enough to have melted extensively. Geochemical, experimental, petrological and geodynamic modelling now show that when the continental crust melts the consequences are crustal-scale. The combination of melting and regional deformation is critical: the presence of melt on grain boundaries weakens rocks, and weak rocks deform faster, influencing the way mountain belts grow and how rifts propagate. Tectonic forces also drive the movement of melt out of the lower continental crust, resulting in an irreversible chemical differentiation of the crust.