The accumulation of sizeable volumes of magma in the upper crust may produce plutons and/or result in supereruptions. Geophysical observations provide constraints on the rates, volumes, and melt distributions in magmatic systems, but they suffer from limited resolution and inherent nonuniqueness. Different, yet complementary, geophysical approaches must be combined with petrological, laboratory, and geochemical measurements. We summarize the results from such a combined approach from the central Andes. Taking a global perspective on large silicic systems reveals that several have >10% partial melt over large areas (10s of km2), and there may be localized zones with 50% or more.