This thesis examines the application of the existing WTO disciplines to the trade-distortive practices carried out by STEs. The hypothesis confirmed by this thesis is that the WTO legal framework is not adapted to the hybrid nature of STEs. This research focuses on the practices developed in the energy sector where the presence of STEs is predominant, providing the perfect stage for its technical and analytical study. Part I sets the stage by describing the important role of STEs in the energy market. Part II provides an answer on how to attribute the activities conducted by STEs to a WTO Member. Part III analyses the treatment granted to STEs' activities under the WTO Agreement. Part IV discusses the main problems arising from applying the existing WTO rules to the activities of STEs. Part V looks at solutions for these problems, found in different forums such as Regional Trade Agreements. The last Part provides recommendations to filling the gaps in the WTO Agreement.