Sustainability assessment is the core instrument for implementing sustainable development targets. The energy sector is playing a crucial role in achieving sustainable development in the country, as energy consumption is the main driver of economic growth; it also has the biggest environmental impact, and therefore sustainability assessment in the energy sector requires special attention. Sustainability Assessment in the Energy Sector consists of seven chapters: in the first chapter, the sustainability concept is described; in the second chapter, sustainability assessment concepts are reviewed and discussed; in the third chapter, sustainability assessment tools and methods are summarized and described; in the fourth chapter, the main problems of sustainable energy development are addressed; in the fifth chapter, externalities in the energy sector are introduced; in the sixth chapter, sustainability assessment tools applicable to the energy sector are categorized; and finally, in the seventh chapter, practical examples of sustainability assessment in the energy sector are provided by covering sustainability assessment of electricity and district heat generation technologies. These methods include the assessment of external costs of electricity generation, sustainability assessment of electricity and transport technologies based on the GHG emission costs and carbon intensities of these technologies. The final chapter also discusses the sustainability assessment of policies and measures for energy sector development scenarios. The book was prepared by summarizing research findings in the field of sustainability assessment in the energy sector, which was conducted by the author of this book over a period of ten years. (Nova)