This chapter includes an investigation on a power smart grid with multiple suppliers and consumers. The first section is devoted to the corresponding model description. The power load flow estimation is presented in a form equivalent to a link-flow traffic assignment problem. The second section concentrates on the competition of consumers, while the third section concentrates on the cooperation of suppliers. From mathematical perspectives, both defined models of economic interaction imply bi-level optimization problems. Thus efficient computational algorithms are required. The last section is devoted to pricing mechanisms for transmission networks with multiple suppliers and consumers. Possible techniques for the arising complex problem of transmission cost sharing are proposed.