Publisher Summary This chapter discusses the neutral-neutral interactions. The thermal energy interaction of neutral particles that are in their ground states is characterized by several features that distinguish it from many collision processes occurring in charged-particle scattering. First of all, at thermal energies the interaction can be described by a potential function that depends only on the relative distance and, in the case of molecules, also on the relative orientation of the partners. This comes about since the relative motion of the nuclei is adiabatic with respect to that of the electrons. The other important feature is that the masses, energies, and potential ranges are generally such that quanta effects are small, and therefore semiclassical and often even classical approximations can be used to explain the observed phenomena. Low-voltage arcs and discharges of the focused electron type are suitable sources for producing fast ions that can be converted into fast neutrals by charge transfer. The neutral beam intensity can be measured with thermal detectors that respond to the energy flux of the beam, with detectors that collect secondary electrons produced by bombardment of metal surfaces by the fast neutrals, or, if the signal-to-noise ratio can be made high enough, by detectors that ionize the beam by electron bombardment. Total elastic cross sections can be measured as a function of energy by attenuation experiments. If the beam-detector geometry and the intensity distribution of the neutral beam are known, a proper analysis of the energy dependence of the total cross sections obtained from such experiments leads to reliable value of the intermolecular potential over a relatively large range of internuclear distance.