In this chapter we define Network Functions Virtualization, including examination of the ETSI model for NFV, as well as NFV Orchestration and Infrastructure. We consider what functionality can actually be virtualized, including provider edge routers, firewalls, deep packet inspection, and intrusion prevention. We introduce the reader to an open source platform for NFV called OPNFV. We list the major vendors involved in NFV, including their products as well as their target domains. Lastly we look at the relationship between SDN and NFV, describing how SDN can be used to enable and even enhance NFV.