When analysing mergers which risk generating anti-competitive effects, competition agencies must establish whether a transaction generates sufficient anti-competitive harm to either prohibit it or impose conditions to its clearance. The point beyond which sufficient harm is established, and a prohibition decision or the acceptance of remedies are justified, is not always clear or easily predictable. This note provides the backdrop for an OECD roundtable on merger prohibitions and conditional clearances held in November 2016 and lays out the issues for discussion, including the standard of proof for agency decisions, changes in the level of certainty that agencies are requiring today with regard to merger remedies, different thresholds of certainty at different stages of the merger review process, and issues regarding the effectiveness of merger decisions.