The physics and chemistry of an interface between two media differs considerably from that in bulk materials due to differences in dimensionality and atomic arrangement. Of particular interest are the studies on catalysis and artificially-structured components for new technologies, which are giving this field additional importance. This was recognised many years ago and has lead to a booming field of research which has now reached a steady state.

The use of brilliant, well-collimated beams at the ESRF has opened new possibilities. Surface physics is studied at two specially devoted beamlines, ID3 and ID32. Whereas phenomena occurring at interfaces are dealt with at many other beamlines. They are in part presented in other chapters since their results may be better understood in the context of applied research, materials or others.

Here we highlight only a few examples. A real breakthrough was achieved by the inauguration of a sample cell which permits the investigation of surface structures by high-energy X-ray diffraction. The energy was chosen in such a way that the X-rays can penetrate a thick Be wall which allows the vessel to be both evacuated to ultra high vacuum and later pressurised to several atmospheres. The researchers involved in this project call it "bridging the pressure gap". Indeed, in the past, the interaction between gases and surfaces has been studied mainly under reduced pressures, whereas, most catalytic processes occur at atmospheric or higher pressures. Hence, structural characterisation can now be performed under conditions which better reflect those found in real processes.

Grazing-incidence diffraction can be used to study monolayers and other small structures grown or deposited on a surface. In this section, its use is represented by the investigation of the structure of films of organic molecules grown on single-crystal surfaces. Additionally, the recent technique of grazing-incidence small-angle scattering is described and its use demonstrated by the study of the three-dimensional growth of metallic nano-particles deposited on a surface.

Another type of investigation of great importance is the analysis of the interaction of dot-like structures with a substrate. An example of inter-diffusion is presented, for germanium deposited on silicon, studied using X-ray absorption spectroscopy.

Although not represented, there is also a large class of investigations where the distortion of the underlying lattice can be unravelled. Such research gives important input for an understanding of the issue of self organisation at surfaces.