This book gathers the latest advances, innovations, and applications in the field of energy geostructures, as presented by international researchers at the 1st World Congress on Energy Geostructures (WCEG), held in Cluj-Napoca, Romania, on September 3, 2026. Promoted by the International Society for Energy Geostructures (ISEG), it includes topics such as geotechnical and structural engineering, soil and rock mechanics, engineering geology, hydrogeology, energy and mechanical engineering, heat-pump technologies, and renewable-energy integration. The papers were selected through a peer-review process, and are organised into three thematic sections: thermo-hydro-mechanical coupling and soil-structure interaction; design of energy geostructures; and construction, monitoring and operation.