This report details the technical background to the Global Mercury Assessment 2018 that has been developed in response to a decision at the twenty-seventh session of the Governing Council of the United Nations Environment Programme (Decision 27/12 (III.21)) that requested the Executive Director “to provide an update [to the GMA 2013 report] within six years”. The Global Mercury Assessment 2018 (summary for policy-makers) was delivered to the fourth session of the UN Environment Assembly (UNEA4) in March 2019.