Network Function Virtualization (NFV) has gained significant attention from both industry and academia as a transformative approach to telecommunication service provisioning. By decoupling Network Functions (NFs) from physical devices, NFV aims to reduce operating and capital expenses and enhance service agility. The paper discusses the relationship between NFV and complementary fields such as Software Defined Networking (SDN) and cloud computing, and surveys the state-of-the-art in NFV, including key projects, standardization efforts, early implementations, use cases, and commercial products. It identifies promising research directions and addresses open challenges, such as testing, validation, resource management, interoperability, and performance of Virtual Network Functions (VNFs). The paper also proposes a reference business model and discusses design considerations for NFV, emphasizing the importance of network architecture, performance, security, resilience, reliability, support for heterogeneity, legacy support, and scalability. Additionally, it explores the relationship between NFV, SDN, and cloud computing, highlighting their complementary nature and potential for mutual enhancement.Network Function Virtualization (NFV) has gained significant attention from both industry and academia as a transformative approach to telecommunication service provisioning. By decoupling Network Functions (NFs) from physical devices, NFV aims to reduce operating and capital expenses and enhance service agility. The paper discusses the relationship between NFV and complementary fields such as Software Defined Networking (SDN) and cloud computing, and surveys the state-of-the-art in NFV, including key projects, standardization efforts, early implementations, use cases, and commercial products. It identifies promising research directions and addresses open challenges, such as testing, validation, resource management, interoperability, and performance of Virtual Network Functions (VNFs). The paper also proposes a reference business model and discusses design considerations for NFV, emphasizing the importance of network architecture, performance, security, resilience, reliability, support for heterogeneity, legacy support, and scalability. Additionally, it explores the relationship between NFV, SDN, and cloud computing, highlighting their complementary nature and potential for mutual enhancement.