Microstructures of metamorphic rocks reflect metamorphic reactions, as well as minimization of interfacial free energy, leading to the formation of polygonal aggregates, rounded inclusion shapes and idioblastic crystals, depending on the degree of crystal anisotropy and the effect of fluids. Microstructures of some slowly cooled mafic and ultramafic igneous rocks also show evidence of reduction of interfacial free energy, but this effect is minimal in granitoids. Symplectic intergrowths, including myrmekite, are useful for inferring some metamorphic reactions. Similar intergrowths occur in oxide and sulphide minerals. Microstructural evidence is important for inferring metamorphic reactions, but requires care in interpretation. Evidence of parent rock-types can be seen in less deformed metamorphic rocks. Many metamorphic minerals show compositional zoning, as revealed especially by X-ray composition images. Microstructures formed at highest metamorphic temperatures commonly show evidence of partial melting, producing migmatites, some with magmatic microstructures. Melting can also occur in some sulphide rocks.