This publication is a historical summary of progress made in energy cooperation in the Greater Mekong Subregion (GMS) since the beginning of the economic cooperation program in 1992. It highlights a two-pronged and phased approach to develop the GMS power market to realize the full benefits of synchronous operations---the policy and institutional framework to promote power trade and the physical interconnection to facilitate cross-border power transfers. This publication aims to help stakeholders understand GMS members' efforts in power cooperation and to draw attention to crucial issues on regional power trade.