Some ISPs are trying to become part of the online advertising market. Such ISPs either: (i) cooperate with online advertising entities (e.g., ad networks) by providing users’ private information to achieve better ad targeting in exchange for a share of the revenue, or (ii) modify the ad traffic on-the-fly such that they divert part of the online advertising revenue for themselves. This is a very important issue because online advertising is at the core of today’s business model and it fuels many “free” applications and services. We study this behavior using game theory to model the interactions between ISPs and ad networks, and we analyze the effects on the Web caused by ISPs taking part in online advertising. Our results show that if the users’ private information can improve ad targeting significantly and if ad networks do not have to pay a high share of revenue to the ISPs, ad networks and ISPs will cooperate to jointly provide targeted online ads. Otherwise, ISPs will divert part of the online ad revenue for themselves. In that case, if the diverted revenue is small, ad networks will not react. However, if their revenue loss is significant, the ad networks will invest into improving the security of the Web and protecting their ad revenue.