em providers to new business approach to align to the more flexible Internet in order to adapt from a single-service provider to a mul- tiple service provider. This big challenge has been responded by the industry with a programmable all-IP (Internet protocol) architecture delivering voice and multimedia functional- ities-the IP multimedia subsystem (IMS). This technology is competing with Web 2.0 and peer-to-peer technology representing the major competitors in today's technology landscape. Converging of networks drives the evolution of new business models further, like to well-known on-demand model into the recently hyped cloud computing. While each approach has its advantages and disadvantages, those dif- ferent service delivery paradigms compete on the level of cost efficiency for develop- ment time and operation as well as their capacity to create and deliver new services in a fast and automatically manageable way. This article discusses the evolution of the IMS as service delivery layer of the Next Generation Networks (NGN), it challenges under the demand to align the service provisioning between legacy systems and the new Web 2.0 area, up to the evolution of Telecom Service Application Programming Interfaces (APIs) for faster service creation and the placement of those technologies toward the new cloud-enabled business models. 4.2 Service Delivery in Telecommunication Networks 4.2.1 Overview of NGNs In recent years, reconstruction of existing Telecom operator networks into new IP-based networks, that is, migration to NGN is gaining momentum. In the back- ground of this movement, there is the need for both the fixed and mobile operators to lower the network operation cost by turning the legacy network into IP network, and to provide new types of communication service to make differences from competitors. Specially, the Fixed Mobile Convergence (FMC) is expected to be the next revenue source for Telecom operators, because by merging fixed and mobile Enabling Service Delivery in Next-Generation Networks telep