This introductory chapter defines the class of Network materials and provides numerous examples. Network materials are classified based on several criteria. They are divided into thermal and athermal, as a function of the dependence of the fiber behavior on temperature, in dry, embedded or embedding, as a function of the presence of embedded entities (nanoparticles, macromolecular entities) or of an embedding matrix, in crosslinked and non-crosslinked function of the nature of fiber interactions. This classification is used in the book to define categories of mechanical behavior. The chapter closes with an outline of the book.