An ecologically sustainable and socially just energy transition represents a significant opportunity to align human activity with the planetary boundaries. Large-scale renewable energy projects, which are necessary for addressing climate change, can have complex and multifaceted impacts on both nature and societies. As an introduction, the first chapter presents and discusses briefly a number of key sustainability concepts, including the United Nations Sustainable Development Goals, the principles of planetary boundaries, and the social foundation via the doughnut economy framework. It also examines the safe operating zone for the planet and the ways in which these concepts connect to the techno-economical design of energy systems based on renewable resources. Furthermore, the chapter examines the history of industrialization to illustrate the accelerated pace of human-induced change and the necessity for immediate action to avert the adverse effects of global warming and biodiversity loss. Additionally, the chapter elucidates the ongoing transition's implications for the skillset required of an energy engineer and offers insights into this topic.