This document discusses the potential role of impact assessment in supporting sustainable energy policy decisions. It outlines key principles of sustainable development and sustainable energy. Impact assessment is presented as a process that can help operationalize sustainable development and address challenges to decision-making by generating information, facilitating deliberation, and helping structure complexity. The document also examines the characteristics of an ideal sustainability assessment approach for energy policy and assesses a case study on nuclear waste management in Belgium. It concludes that while impact assessment can help address some challenges, interpreting sustainability remains difficult, especially for issues like nuclear energy.