Merger review involves the complex ex-ante assessment of factual, economic and opinion evidence by agencies undertaking complicated, detailed, subjective and forward-looking economic analyses, using limited information and resources. It is inevitable that errors will be made: harmless mergers will be blocked (false positive/Type I errors) while anticompetitive mergers will be cleared (false negative/Type II errors.). The current Irish merger enforcement legislation explicitly allows merging parties to appeal decisions. Non-merging parties must challenge a decision by way of judicial review. The thesis explores whether the status quo for third parties (i.e. judicial review) is sufficient. The deficiencies of the current regime are highlighted, ways in which the legislation and the institutional structure could be amended are developed, and two alternative options (i.e. a statutory judicial review process and a statutory appeals process) are examined. Designing a process for challenging merger decisions requires a balance between an independent and thorough review process and the need for a speedy process which provides legal certainty. The status quo for third parties is found not to strike the appropriate level of balance between the competing objectives of thoroughness, timeliness and certainty. Of the two possible alternatives a statutory appeal defined in the legislation is, in principle, capable of delivering on these objectives. However, its effectiveness is undermined by: (i) the additional costs it imposes on the Competition Authority, merging parties and the courts; and (ii) the continued opportunity for third party appellants under such a process to challenge decisions (even those which were not upheld on appeal) by way of judicial review which generates an unacceptable level of uncertainty. The preferred option is to provide parties with the possibility of challenging substantive merger enforcement decisions of the Competition Authority through a statutory judicial review process. Suggestions on how this process could be drafted in the legislation are provided.