Abstract: Network function virtualization (NFV) is a technology that realizes network functions based on virtualization technology and standard commercial servers, switches, and storage, so as to substitute traditional middleboxes that are dedicated devices. NFV is proposed to save construction and operation cost for network service providers. NFV can also improve flexibility and scalability of network services, and can motivate development and deployment of new network functions. Currently, NFV is still under developing, and problems remain in system performance, management and orchestration, reliability, availability, security, and programmability. There have been a number of studies to address these problems. In this paper, we review the NFV architecture and basic technologies, and summarize the key problems to be solved. Then, we categorize the existing studies by a “four quadrants” method. We analyze and compare a number of typical solutions, and summarize the advantages and overheads. Finally, we discuss the research trends of NFV.