By mapping theories of harm that have been applied in recent digital merger cases and considering the analyses that have already been undertaken, this note explores the question of whether existing theories of harm are well suited or should be further adapted to comprehensively capture competitive harms arising from mergers in these markets. Or alternatively, whether new theories of harms are needed and if so, what they might look like. It was prepared as a background note for discussions on “Theories of Harm for Digital Mergers” taking place at the June 2023 session of the OECD Competition Committee.