What do stained glass, sunscreen, magnetic hard drives, heterogeneous catalysts, consumer electronics, stain-resistant clothing, self-cleaning glass, and medical diagnostics all have in common? All of them derive some special property and utility from nanoscale materials : ordinary elements and inorganic compounds such as gold, silver, TiO 2 , chromium, SiO 2 , and silicon that acquire different properties when their characteristic dimensions are somewhere between 1 and 100 nm . In this chapter we will learn about the basic science of nanomaterials, i.e., what it is about their size that makes them different.