Abstract: Developments in capital markets have fueled a concentration of horizontal ownership across competing firms, and this has been linked to anticompetitive effects and economic underperformance. The debate about such ownership concentration has proven contentious and controversial. This symposium titled “Common Ownership: Illuminating a Great 21st Century Antitrust Debate” brings together key new works on the topic that confirm, extend, and illuminate the prior empirical findings and policy implications. Among other things, these contributions survey the recent empirical literature, provide new important empirical results about the extent and effect of horizontal ownership, offer a methodological critique, highlight concepts that address core capital market and labor market linkages, and articulate ideas for policy development to tackle emerging contingencies.