This book chapter deals with the application of life cycle assessment (LCA) to sustainable energy systems and technologies around the world. It reviews the practical experiences of LCA application to energy systems and their outcomes in the energy sector. However, as the environmental impact of energy systems can be seen in multiple lenses, the focus of this book chapter has been limited to a critical review of LCA application in electricity generation as a central example of energy supply system along with LCA of energy use in built environment from demand perspective. Critical reviews and related case studies have been presented in this book chapter which will be beneficial for researchers and practitioners in the LCA field to advance their expertise in applying LCA methodology in energyrelated technologies. This chapter will also catch the interest of learners/students, as it enables them to have a deep understanding of the diverse environmental impacts from the energy sector, as well as they will get a clear conception of sustainable energy technologies through the comprehensive life cycle analysis.