The relationship between competition law and regulation is a perennial question in competition policy. This article considers what happens when we apply competition law within markets already subject to a degree of alternative regulatory intervention – and, more specifically, how the presence of regulation can inform the antitrust assessment exercise. First, it asks why we might need, but also be reluctant, to apply competition law to market problems arising in regulated sectors. It then examines the potential impact of regulation within the task of competition law assessment from three overlapping perspectives: (1) where regulation forms an integral part of the ‘legal and economic context’; (2) where a regulatory standard is applied to set the boundaries of competition on the merits; and (3) where competition intervention functions as a means of ‘course correction’ to the underlying regulatory regime.