A large number of organic and inorganic solids have crystal structures in which the fundamental structural units form linear chains. While most of these materials are insulators or semiconductors; several groups have partially filled electron bands, and consequently display metallic behavior at high temperatures. A variety of inorganic materials have strongly anisotropic crystal, and consequently strongly anisotropic electronic structures; which often display various transitions from metallic to nonmetallic states. Among the many groups of such materials, three have been explored in detail and these materials are by now well established examples of the charge density wave ground state. The materials which belong to the different groups do indeed show metallic conduction down to low temperatures. Among the different materials which display a variety of phase transitions, only the simplest examples of density wave formation are discussed. The materials are relatively good metals along the chain direction with room temperature conductivities parallel to the chains.