Quantification of harm is regarded as one of the most significant obstacles for the full compensation of harm and development of private enforcement within the European Union, including CEE Member States. Consequently, the Damages Directive establishes general rules and requirements for the quantification of harm, such as a rebuttable presumption of harm in case of cartels, the power of national courts to estimate harm as well as others, which closely interact with the principle of full compensation emphasized by the case-law of the European Union and directly established in the Damages Directive. The main focus of this paper is the effectiveness of the rules on the quantification of harm in general, and how these rules will contribute to the development of private antitrust enforcement in CEE Member States. Therefore, one of the issues to be discussed in the paper is the analysis of how, and to what extent specific rules and requirements for the quantification of harm have been transposed into the national legislation of CEE Member States. As certain CEE national jurisdictions have had certain rules for the quantification of harm already before the implementation of the Damages Directive, the paper analyses how effective these rules have been, and how much they have contributed to the development of private antitrust enforcement of those CEE national jurisdictions. Previous experience of those CEE Member States in applying specific rules for the quantification of harm is important, in order to assess the possible impact of the newly introduced rules on the quantification of harm and on private antitrust enforcement in general in other CEE Member States. The rules for the quantification of harm will not enhance private antitrust enforcement on their own, however, their effective application by national courts together with other rules under the Damages Directive should contribute to a quicker development of private enforcement in CEE Members States.