The development of polymer nanocomposites has been an area of high scientific and industrial interest in the recent years, due to several improvements achieved in these materials, as a result of the combination of a polymeric matrix and, usually, an inorganic nanomaterial. The improved performance of those materials can include mechanical strength, toughness and stiffness, electrical and thermal conductivity, superior flame retardancy and higher barrier to moisture and gases. Nanocomposites can also show unique design possibilities, which offer excellent advantages in creating functional materials with desired properties for specific applications. The possibility of using natural resources and the fact of being environmentally friendly have also offered new opportunities for applications. This chapter aims to review the main topics and recent progresses related to polymer nanocomposites, such as techniques of characterization, methods of production, structures, compatibilization and applications. First, the most important concepts about nanocomposites will be presented. Additionally, an approach on the different types of filler that can be used as reinforcement in polymeric matrices will be made. After that, sections about methods of production and structures of nanocomposites will be detailed. Finally, some properties and potential applications that have been achieved in polymer nanocomposites will be highlighted.