We present Sunlight, a system that detects the causes of target-ing phenomena on the web – such as personalized advertisements, recommendations, or content – at large scale and with solid statisti-cal confidence. Today’s web is growing increasingly complex and impenetrable as myriad of services collect, analyze, use, and ex-change users ’ personal information. No one can tell who has what data, for what purposes they are using it, and how those uses affect the users. The few studies that exist reveal problematic effects – such as discriminatory pricing and advertising – but they are either too small-scale to generalize or lack formal assessments of confi-dence in the results, making them difficult to trust or interpret. Sunlight brings a principled and scalable methodology to per-sonal data measurements by adapting well-established methods from statistics for the specific problem of targeting detection. Our method-

Content and services which are offered for free on the Internet are primarily monetized through online advertisement. This business model relies on the implicit agreement between content providers and users where viewing ads is the price for the “free ” content. This status quo is not acceptable to all users, however, as mani-fested by the rise of ad-blocking plugins which are available for all popular Web browsers. Indeed, ad-blockers have the potential to substantially disrupt the widely established business model of “free ” content—currently one of the core elements on which the Web is built. In this work, we shed light on how users interact with ads. We show how to leverage the functionality of AdBlock Plus, one of the most popular ad-blockers to identify ad traffic from passive net-work measurements. We complement previous work, which fo-cuses on active measurements, by characterizing ad-traffic in the wild, i.e., as seen in a residential broadband network of a major European ISP. Finally, we assess the prevalence of ad-blockers in this particular network and discuss possible implications for con-tent providers and ISPs.