The General Court’s judgment in CK Telecoms is of major importance. On matters of substance, the Court explains for the first time how to apply the substantial impediment of effective competition (SIEC) test to mergers which do not create or strengthen a dominant position. The judgment calls into question certain aspects of the Horizontal Merger Guidelines. On matters of procedure, the Court sets out the standard of proof in merger control and reveals itself willing to examine the Commission decision in depth. However, it is argued that the Court’s disagreement with the decision is not always expressed convincingly. This episode reveals the need for the Commission to explain its theory of harm more clearly and the need for the General Court to explain its conclusions more fully. The legislator may see this episode as an occasion to consider the suitability of assessing mergers based on the application of ever more complex economic analysis.