Abstract: This Article shows that new economic proofs and empirical evidence provide powerful confirmation that, even when horizontal shareholders individually have minority stakes, horizontal shareholding in concentrated markets often has anticompetitive effects. The new economic proofs show that, without any need for coordination or communication, horizontal shareholding will cause corporate managers to lessen competition to the extent they care about their vote share or re-election odds and will cause executive compensation to be based less on firm performance and more on industry performance. The new empirical evidence consists of cross-industry studies which confirm that, just as the proofs predict, increased horizontal shareholding increases the distortion of executive compensation and the gap between corporate profits and investment. I also provide new analysis demonstrating that critiques of earlier empirical studies showing adverse price effects for airlines and banking are generally invalid and that addressing the valid subset of those critiques actually increases the estimated price effects. I further demonstrate that the various excuses for delaying enforcement action are meritless. Finally, I provide new legal theories for tackling the problem of horizontal shareholding. I show that when horizontal shareholding has anticompetitive effects, it is illegal not only under Clayton Act §7, but also under Sherman Act §1. In fact, the historic trusts that were the core target of antitrust law were horizontal shareholders. I further show that anticompetitive horizontal shareholding also constitutes an illegal agreement or concerted practice under EU Treaty Article 101, as well as an abuse of collective dominance under Article 102.