This paper discusses some of the challenges algorithms present for both competition law enforcement and market regulation. In particular, the paper addresses the question of whether antitrust agencies should revise the traditional concepts of agreement and tacit collusion for antitrust purposes, and discusses how traditional antitrust tools might be used to tackle some forms of algorithmic collusion. Recognising the multiple risks of algorithms and machine learning for society, the paper also raises the question of whether there is need to regulate algorithms and the possible consequences that such a policy choice may have on competition and innovation. It was prepared as a background note for a discussion held at the OECD in June 2017 on algorithms and collusion.