This chapter is concerned with horizontal cooperation agreements which the competition authorities in the EU and the UK may be prepared to countenance. There may be circumstances in which competitors cooperate with one another in a way that delivers economic benefits, not just for themselves, but for consumer welfare as well. After a discussion of joint ventures, the chapter discusses the application of Article 101 to horizontal cooperation agreements. It refers, in particular, to the European Commission’s Guidelines on Horizontal Cooperation Agreements and discusses, in turn, information exchange, research and development agreements, production agreements, purchasing agreements, commercialisation agreements, standardisation agreements, sustainability agreements and other cases of permissible horizontal cooperation. It concludes with a brief look at the treatment of horizontal cooperation agreements under UK competition law.