How can members of the DAC effectively tackle critical environmental challenges and threats - such as climate change, pollution, and loss of soil fertility and biodiversity - in activities supporting the 2030 Agenda in developing countries? Most already have environmental safeguards in place to screen out negative environmental practice, but they need far more robust policies, capacities and approaches for mainstreaming environment across all their development co-operation activities. This report examines five critical areas for mainstreaming: strong policy commitment and leadership; robust systems, processes and tools; capacity and continuous skill development; shared knowledge, learning and engagement; well-supported country systems. On that basis, the report suggests priority actions for the OECD-DAC, its Network on Environment and Development (ENVIRONET) and the wider development community.