This paper focuses on online display advertising, whereby publishers display advertisements on their website against remuneration. This form of advertising represents a critically important source of revenues for publishers, from large news organisations to online game producers to blogs, offering valuable content to Internet users. Given the importance of online display advertising to publishers, it is no wonder that this area has been the subject of intense discussion among stakeholders and has raised the attention of competition authorities. In particular, concerns have expressed that publishers do not receive their fair share of advertising revenues due to the large fees that are captured by the “ad tech” companies intermediating between advertisers and publishers. This paper provides an overview of the online display advertising landscape, and explores whether Google, the leading ad tech providers, has engaged in potential exclusionary and exploitative conduct.