Network functions virtualization (NFV) is consolidating as one of the base technologies for the design, deployment, and operation of network services. NFV can be seen as a natural evolution of the trend to cloud technologies in IT, and hence perceived as bringing them to the network provider environments. While this can be true for the simplest cases, focused on the IT services network providers rely on, the nature of network services raises unique requirements on the overall virtualization process. NFV aims to provide at the same time an opportunity to network providers, not only in reducing operational costs but also in bringing the promise of easing the development and activation of new services, thereby reducing their time-to-market and opening new approaches for service provisioning and operation, in general. In this chapter, the authors analyse these requirements and opportunities, reviewing the state of the art in this new way of dealing with network services. Also, the chapter presents some NFV deployments endorsed by some network operators and identifies some remaining challenges.