The purpose of demand side integration (DSI) programs is to modify the behavior of electrical loads of different customer types to minimize energy costs, enhance energy utilization or system reliability, or match utilization to environmental factors, such as availability of renewable energy. When planned accordingly, they can also contribute to defer investment in new infrastructure by diminishing the peak capacity requirements of the system. This chapter is focused on all forms of DSI with a demand response objective. It does not address the issue of end-use efficiency, although this latter aspect is recognized as being fundamentally important to achieve reduction of demand and improved environmental performance. The chapter discusses drivers and various forms of DSI that are being utilized in different countries. It provides an overview of market structures that affect adoption of DSI, changes in demand side management because of market changes, implementations of DSI, and drivers and barriers. The roles, responsibilities, and potential economic drivers of network operators to support or contribute to the implementation of DSI initiatives are examined. The extent to which regulatory support or economic mechanisms (or incentives) might be required for network operators to undertake such responsibilities is considered. General considerations on distribution and transmission networks are provided together with some discussions on the different types of system operators, network grid owners, and other participants related to distribution and transmission networks. These considerations are necessary to understand the complexity of power systems following deregulation and its possible impact on the development of DSI from the network perspective. The different kinds of services that might be provided by DSI to the different power system participants are described, along with the roles and responsibilities that might be endorsed by distribution system operators and transmission system operators. The barriers to DSI development and the need for appropriate regulatory support and economic mechanisms for the involvement of distribution system operators (DSOs) and transmission system operators (TSOs) in the implementation of DSI initiatives are also examined.