The water-energy nexus has increasingly been recognized as one of the key factors underlying the sustainability and security of future water and energy supply. An improved understanding of this issue is required to guide political decision-making on the choice of technologies that can better lead towards water-energy efficient scenarios. One of the biggest challenges relies on the reduction of uncertainties over selected variables, and the analysis of trends and interrelations that may have an impact on the future of water and energy security. This paper presents the results of a Delphi study on prospective and future trends of the water-energy nexus and energy technologies. Based on the opinion and contributions of experts on the topic from different backgrounds, institutions and disciplines, the results indicate the importance of technology innovation and transfer as the main conditioning factors to achieve energy and water security. In terms of energy, biofuels and shale gas are perceived to have the highest potential impacts on water quantity and especially on water quality. Thus their evolution will be influential for future water and environmental security. Biofuels in particular, together with the rising demands for food, have the highest prospects for an increase in agricultural water withdrawals.