This chapter discusses inelastic electron scattering. Electron energy loss spectroscopy (EELS) has been developed to the degree of sophistication required for the measurement of surface phonon dispersion curves. Surface phonons of relatively high energy can be investigated with EELS, so that not only Rayleigh modes, but also surface modes located in higher lying gaps in the bulk phonon spectrum as well as surface modes associated with adsorbed species can be studied. The chapter also discusses correspondence of units and theoretical models for surface phonons. There is no single unit that has achieved universal acceptance for expressing the frequency of surface phonons. Ordinary frequency in THz, angular frequency in 1013 rad s-1, wave number in cm-1, and energy in meV have all been used. Energy conversion factors are listed. The broad range of crystal surfaces for which surface phonon dispersion curves have been determined experimentally has entailed the use of a corresponding large range of theoretical models in their interpretation. The difficulty of theoretical analysis varies significantly from one class of materials to another. In the case of strongly ionic materials such as the alkali halides, the forces of interaction between ions are dominated by Coulomb, polarization and quantum mechanical repulsive interactions. The force constants coupling atoms at and near the surface are frequently modified in order to reproduce experimental surface phonon data. Unfortunately, there is no consensus on a satisfactory model for the bulk crystal in many cases.