The Grand Chamber set aside the judgment of the General Court in CK Telecoms having found eight errors in law. This paper dissects the judgment of the Grand Chamber to illustrate and explain the differences between the two judgments as well as the significance of these differences. The Grand Chamber clarifies the standard of proof and confirms that the analysis of unilateral effects in oligopoly markets requires a holistic analysis of a number of factors none of which is decisive. While it ratifies the analytical stance of the Commission this does not necessarily indicate that the Commission will prevail as the appeal is heard again at the General Court. Furthermore, the judgment as well as the new techniques used by the Commission suggest that a review of the Horizontal Merger Guidelines is much needed More generally, the clash between the two courts reveals a fundamental difference between judges in the two courts about the role of EU competition law.