The private sector can be a strategic partner in building and maintaining transmission networks, depending on how well its participation is structured to achieve the desired outcomes. In Peru, private sector participation has driven down both investment and operation and maintenance (O and M) costs. Upper-middle-income countries account for about 65 percent of the private capital raised for investment in transmission infrastructure between 1994 and 2013. In the presence of the right policies and investment climate, private sector participation in transmission systems can improve the security, reliability, and quality of supply in a cost-effective manner. Strong, independent regulatory institutions are critical. Such institutions should be equipped to develop detailed expansion plans that make it possible to identify and select projects.