This thesis presents a novel policy assessment tool that serves as a decision support framework for analysis of energy policy problems involving multilevel, multidimensional and multi-stakeholder complexity. This new approach, The Energy Policy Assessment Technique (E-PAT), integrates System Dynamics Simulation and Multi-Criteria Evaluation models into a unified assessment framework. The E-PAT doubles as a systems-thinking model and a multi-criteria evaluation archetype. The E-PAT is designed for assessing policy issues, particularly those relating to energy planning in a national context. It is a suitable tool for evaluation and selection of optimal policies from competing and conflicting alternatives. To test its robustness and practical application, a case study on energy policy evaluation in Nigeria is carried out with this tool. A model of the Nigerian energy economy was constructed, and three proposed government policies for Sustainable Energy Development were evaluated relative to a Business-As-Usual case. The evaluation process identified ‘best’ policy options according to stakeholder viewpoints. The original contribution of this research is the creation of an integrated, transparent and comprehensive assessment tool, and the development of measurable Energy Sustainability Indicators (ESI), to aid policymakers in diverse policy analysis. The new tool is intended to support assessment of energy policies in relation to impacts on the economy, society and environment.