prep­ aration, (ii) control of the surface's environment, (iii) improve­ ment of measurement tools and techniques, and (iv) the importance of surface properties in many new industrial areas. Nearly eighty techniques are now available to help us answer to the following questions: what is the surface structure or arrangement of surface atoms? what are the atomic species present? what is the spatial distribution of foreign atoms? what are the nature and distribution of possible defects on the surface? what is the electronic structure of the surface atoms? what is the motion of atoms on the surface? In general, two or more analytical techniques are used concurrently to assure unequivocal answers to problems. Different techniques employ different combina. tions of incident probes and the scattered or secondary particles that convey information regarding the sur­ faces.