This report captures lessons learned through PEI experience over its 13 years of existence and is divided into three main sections: Overview: PEI 2005–2018 briefly reviews PEI’s conceptual and programmatic approach to poverty-environment mainstreaming Looking Back synthesizes 10 lessons learned and good practice from PEI country-level experience in mainstreaming poverty-environment objectives into national, sectoral and subnational development planning, implementation and monitoring systems and processes Looking Ahead provides a snapshot of how the integrated approaches, tools and lessons from PEI experience can be leveraged to help strengthen and accelerate national SDG implementation, including through the successor to PEI, the new joint UNDP–UNEP programme Poverty-Environment Action for Sustainable Development Goals (2018–2022).