Over the last few years, online advertising has moved from a data-supported to a data-driven industry. Data has shifted from the periphery to the centre of business operations. The use of data now takes place in a highly networked and complex environment consisting of thousands of market participants with dozens of different roles and purposes. New acronyms for specialized services in the process of delivering advertising to the consumer are rampant, creating a highly intransparent and fluid ecosystem whose composition changes in near real-time. This presentation seeks in a first step to define core concepts that constitute this ecosystem in order to provide relevant and reliant coordinates for grasping online advertising today. It then introduces distinct types of market participants, how they have evolved, what their tasks and business strategies are. Within this framework, the presentation continues to describe the modalities of data flows within online advertising, how and which data is collected by whom, how data is exchanged and packed, for what purposes and what business outcomes data generates. The presentation then explores how data has become a competitive differentiator that affects not just online advertising, but the media business more comprehensively. Lastly, it relates cases of M&A activity and business strategies to market shares and highlights how new tendencies of market concentration are emerging globally and in Europe. It concludes with a proposition that issues of media pluralism do not just apply to the 'interface' - what can be seen on a screen by the consumer - but also to the underlying 'infrastructure' of data and delivery, which while hidden from consumers' view, co-determine the future make-up of the media landscape.