In the decade since its invention, the Scanning Tunneling Microscope (STM) has made deamatic impact in fields as wide ranging as materials science, semiconductor physics, biology, electrochemistry, tribology, biochemistry, surface thermodynamics, organic chemistry, etc. As the technique becomes more widely available its use is spreading to even more divers fields such as catalysis, micromechanics, and medical implant technology. The reaseon for its nearly instantaneous acceptance as a characterization tool is that STM provides three-dimensional, real- space images of surfaces at high spatial resolution. When the sample is clean and flat, even atoms can be imaged. This remarkable capability is illustrated in the image of the InSb (110) surface shown in Figure 1.1 in which the periodic rows of bump-like features associated with surface atoms are apparent. This capability will be further demonstrated throughout the book. In the case of some materials, such as biological tissue and large organic molecules, STM allows imaging at unprecedented levels of resolution without resorting to conditions which destroy the samples. This is in contrast to other structural characterization techniques, including electron microscopy (scanning electron microscopy and transmission electron microscopy), which require relatively high vacuum conditions. In the case of crystalline solids high resolution transmission electron microscopy can be used to examine bulk atomic structure; however, does not provide the local information about bonding that is obtained from tunneling spectroscopy. Thus, the STM brings a complimentary probe of atomic structure and local properties to the arsenal of caharacterization tools.