Surface science is the study of chemical reactions and physical processes (e.g., phase transitions) that occur at surfaces and interfaces. Surfaces are WHERE THE ACTION IS, the "seat of communication between two phases." Atomic level characterization of solid surfaces is important for understanding and tailoring properties of nanoparticles and functional nanostructures. This is because of the strong "surface”—“nanoparticle” connection. There are two aspects to this: 1) surface -properties of- nanoparticles, and 2) nanoparticles -on- surfaces. By necessity, nanometer scale objects have a large part of their material present at the surface, and these atoms have different and unknown chemical and physical properties. Also, nanoparticles and nanometer-scale devices and structures are often deposited or constructed at a solid interface, and thus the surface properties and chemistry of that interface can control the construction, stability, and properties of the particle or device. Surface science is also at the heart of an enormous range of industrial processes, from heterogeneous catalysis in the chemical and petroleum industries to microelectronic device fabrication and characterization. Chemistry at surfaces controls hazardous gas sensors, adhesion, corrosion, and lubrication. Fields as esoteric (but exciting) as molecular robotics hinge on advances in surface science.