A margin squeeze is an exclusionary form of abuse of a dominant position that a vertically integrated firm can implement when it sells its upstream bottleneck input to its downstream competitors. Because it is vertically integrated, the dominant incumbent can reduce the margin between the input price charged to competitors and the retail price charged to end-users by either raising the price of the input and/or lowering the price of its retail product/services to such an extent that the remaining margin of profit is insufficient for its rivals to remain competitive. Although the scenario of margin squeeze seems to be rather simple, the underlying economic and legal theories are not. Consequently, detecting a margin squeeze requires competition authorities to apply a complex imputation test, which in turn requires various methodological choices that can determine the outcome of the investigation. The principal purpose of the dissertation is to determine whether the European Commission's margin squeeze decisions are consistent with EU case law. The dissertation examines two alternative hypotheses. Under hypothesis A, margin squeeze is presented as a deviation from the essential facilities doctrine, which could be seen as an expression of regulatory competition law. Hypothesis B assumes that it constitutes another form of vertical foreclosure, the main question then being under what exact conditions foreclosure is likely in network industries where the margin squeeze doctrine traditionally applies. Two conclusions follow from the analysis. First, margin squeeze constitutes another theory of vertical foreclosure, and accordingly cannot be seen as an unjustified deviation from refusal to deal and essential facilities cases. Second, to ensure that the theory of harm in margin squeeze cases is credible, competition authorities could enhance their current analytical framework by regularly reviewing various additional elements, in particular the extent to which the wholesale product is important for downstream competition.