Two phases or materials meet at an interface. Heat, mass, and charge transfer occur at interfaces as do chemical reactions. In the solid phase, materials can undergo abrupt changes in composition, structure, and bonding at interfaces. Consider the gate region of a transistor in a computer chip in which a dielectric material that is amorphous to avoid grain boundaries is bonded to a semiconductor that is monocrystalline to enhance charge flow. What does the transition region look like between an amorphous metal oxide and a semiconductor alloy such as silicon-germanium used in current computer chips? How many atomic layers does the transition consume? Is the bonding of all of the atoms in both materials satisfied? And can the process used to make the interface improve transistor performance? We will show that that the transition region is only about one nanometer and with germanium that sulfur atoms chemically and electrically passivate dangling bonds at the interface. Generalizing this idea one can start to think about how to turn on and turn off the chemistry on surfaces for other applications. We will show that self-assembled monolayers prevent III-V surfaces from oxidizing, that copper nanoparticles covered with an ionic liquid bond to one another at low temperatures forming an electrically conductive film, that nanoparticle silver may be a better partial oxidation catalyst, and that II-VI quantum dots self-assemble into nanowires and webs.