Most of the entropy of materials comes from vibrations of atoms -- vibrational entropy is typically an order-of-magnitude larger than other sources, such as configurational entropy. Historically, differences in vibrational entropy between different phases have been subtle and troublesome to assess. Some trends and rules emerged over the years, such as how the formation of short, stiff bonds tends to reduce the vibrational entropy. The situation at high temperatures is complicated, but arguably more important for materials processing. At elevated temperatures, the harmonic and quasiharmonic approximations are unreliable. All materials have phonon-phonon interactions at high temperatures because interatomic potentials are not perfectly harmonic. Metals also have electron-phonon interactions, and magnon-phonon interactions are important for iron, for example. For less-complicated materials, it is exciting that we can now measure or calculate accurately the different parts of entropy at elevated temperatures, even when the material is far from a harmonic solid.