The study deals with the legal implications and core meaning of environmental sustainability. Ecological sustainability is one of the three pillars of sustainability (the others are economic and social sustainability). By giving an in-depth analysis of the entire legal background of environmental sustainability, the study reveals the relevant normative basis of this concept in international law. Furthermore, it focuses on the implementation of the 2015-2030 United Nations Sustainable Development Goals (UN SDGs or SDGs) as well. The SDGs are policy-based general commitments rather than binding and enforceable norms. However, their impacts are relevant to the findings of the United Nations member states’ policies. Nevertheless, the goals and the indicators and targets attached to the goals are crucial to achieving sustainability. The study examines the legal status as well as the legal relevance of the goals (and targets and indicators) of the eco-friendly SDGs. Then, the environmental sustainability issues will be analysed in an in-depth way. Bearing in mind the generally poor (or poorer than expected) state implementation of the SDGs, the study reveals the monitoring and review process of the SDGs.