This chapter focuses on the challenges of developing deep geothermal projects in a cross-border context. It highlights the legal obstacles and levers to the development of this renewable energy by questioning, on the one hand, the role of the law in defining acceptable risk – is it possible to develop this type of projects with a high sustainable energy potential while minimising health and environmental risks? – and by questioning, on the other hand, the place of environmental law in a cross-border context – how are the possible environmental effects of a project in a cross-border region such as the Upper Rhine taken into account?