TEAS is an exclusively surface-sensitive technique which can be employed to investigate both structure and dynamics of solid surfaces. In most cases, helium atoms are used (hence HAS) as probe particles. The beam is formed by expansion of the helium gas under high pressure (typically 200 bar) into the vacuum chamber: this requires the usage of powerful vacuum pumping systems. Due to the beam energies of about 7-100 meV, the interaction with the surface is so weak that TEAS can also be applied to weakly bound adsorbate systems like physisorbed layers. Using high-resolution time-of-flight techniques the surface dynamics can be observed with an energy resolution of about 0.1 meV whereas various diffraction methods enable the investigation of the surface morphology after or during epitaxy.