Chapter 13. Electric Power Markets in Transition: Agent Based Modeling Tools for Transactive Energy Support Electric power systems consist of large numbers of heterogeneous participants interacting within an intricate layered network of economic and operational relationships. Decision making in these systems has been extensively decentralized in many industrialized countries over the past twenty years in an attempt to increase their reliability and efficiency. Given the high negative impact of power disruptions these decentralization efforts have typically been preceded by extensive sensitivity studies with empirically based computational models. This chapter discusses the current and potential use of agent based computational modeling to develop novel transactive energy system (TES) designs for electric power systems. TES designs are decentralized market based designs that permit electric power systems to operate more fully in accordance with basic economic principles while maintaining overall system reliability and efficiency.