SURFACES ACT AND PERFORM DIFFERENTLY FROM the bulk material because they have two distinct properties that do not exist in the bulk. First, the unbalanced forces on molecules or atoms at the surface cause it to have excess free energy when compared to the bulk  (i.e., they are in a higher state of potential energy). This excess energy per unit area is the thermodynamic cost of forming a new surface; it has the units of energy/area or force/length. The latter units are those of tension, and the surface excess free energy is often known as surface tension and can be measured for liquids as such. (A more detailed discussion of this excess free energy identified with a surface, including the thermodynamic definition of surface tension, is given below.) Second, there is a preferred direction normal to the surface that can be defined in the mechanical definition  of surface tension. This directional effect of surfaces can be observed in molecular directionality in adsorption phenomena, surface area minimization by surface tension effects, and molecular ordering in Langmuir-Blodgett films deposited from ordered mono-molecular layers (monolayers). Since the first version of this chapter was published in 1995, there has been a significant increase in the attention given to surface and interfacial effects, including those in organic coating systems . Much of this interest has been due to the increasing importance of science at the nanoscale. When one approaches nanometer dimensions, surface effects begin to predominate, and careful consideration must be given to surface energetics . For this reason, we have revised and updated this chapter into its present form. Much of what was stated in the first version remains true, but more recent research has yielded additional knowledge regarding the science of surfaces. Scanning tunneling microscopy, atomic force microscopy , and a broad range of new characterization tools that greatly enhance our capabilities to characterize and visualize surfaces at nm dimensions have become everyday tools for scientists and engineers working in many fields. Surfaces occur only when the energetic cost of creating an interface between two dissimilar materials or phases is satisfied. Thus, there are no stable interfaces between gases, nor among phases in a material above its critical point. However, surfaces can occur between gas and liquid (g/l), liquid and liquid (l/l), gas and solid (g/s), liquid and solid (l/s), and solid and solid (s/s) phases. An example of a g/l interface is the surface of an open, unfrozen body of water. Because a liquid surface cannot support a stress, its surface energy phenomena can be analyzed directly by equilibrium thermodynamics. For a solid, object history will affect the properties of the surface. As a result, surface energy and surface stress must both be considered in analyzing the properties of an interface . Even though this is well known, there has not been as much concern given to the surface history of solids when determining their surface properties as is necessary .