This PhD is an examination into some of the costs and benefits arising from the application of the Fair Trading Act 1973 and the Competition Act 1980 to single firm anti-competitive conduct and complex monopoly conduct in the United Kingdom. The theoretical arguments advanced for the application of competition policy generally, along with the costs identified as likely to flow from this policy, are examined in an attempt to devise a criteria by which the application of competition policy in specific casesm ay be assessedE. nforcement activity of the Office of Fair Trading (OFT) and Monopolies and Mergers Commission (MMC) is examined to consider the extent to which previous actions have resulted in outcomes that may be identified or measured. Three specific investigations conducted between 1993 and 1997 are examined in some detail in ChaptersS , 6 and 7. Thesea rc related in Chapter 8 to more general experienceso f thosei nvolved repeatedlyw ith the operation of the regimei n the United Kingdom. The experiencesa nd evidence drawn together in these four chapters have not, to the author's knowledge, previously been so considered or set out. It is shown that the mechanism by which the policy is put into effect is heavily criticised, and that there are aspects of the procedure that impose burdens beyond those necessary to achieve the given result. In this context the experience of the American regime is used as a comparative example. The work concludes with a synthesis of the problems identified, and offers some possible solutions to the difficulties raised by the regime at the time of writing. Some consideration is taken into account of the future shape of the regime following the entry into force of the Competition Act 1998, although much of the work presented here remains valid to the operation of the new regime.