This year, UN Environment will publish the tenth edition of the annual Emissions Gap Report. To mark the 10-year anniversary and as a contribution to the United Nations Secretary-General's Climate Action Summit, this publication revisits the gap rationale and how it has evolved, comparing the expectations following the Copenhagen Accord with the reality 10 years later. The findings are sobering. Despite a decade of increasing political and societal focus on climate change and the milestone Paris Agreement, global greenhouse gas (GHG) emissions have not been curbed, and the emissions gap is larger than ever. The challenges for the United Nations Secretary-General’s Climate Action Summit and for international climate change negotiations in 2019 are clear. Unless mitigation ambition and action increase substantially and immediately in the form of new or updated nationally determined contributions (NDCs) by 2020 and are reflected in ambitious long-term GHG development strategies, exceeding the 1.5°C goal can no longer be avoided, and achieving the well-below 2°C temperature goal becomes increasingly challenging. These and other key lessons emerging from a decade of Emissions Gap Reports are summarized under the 10 headings of this publication. 1. The Emissions Gap Report - the annual gauge of the disconnect between where we are and where we need to be -- 2. A decade lost - essentially no change in global emissions trend -- 3. The emissions gap is larger than ever -- 4. The global challenge - the ambition level of current NDCs needs to be tripled to get on track to 2°C and increased fivefold to align with 1.5°C -- 5. The gap can still be bridged, but unprecedented and immediate action is required -- 6. Decarbonizing energy supply and transport is key for transformational change -- 7. Phasing out coal is indispensable, but requires a balanced transition -- 8. Nature-based solutions can make a large contribution and are currently the main option for CO₂ removal -- 9. Non-State and subnational actors are essential, but the current mitigation impacts are still limited and poorly documented -- 10. Innovation and new solutions are needed for long-term carbon neutrality.