Targeted advertising largely contributes to the support of free web services. However, it is also increasingly raising concerns from users, mainly due to its lack of transparency. The objective of this proposal is to increase the transparency of targeted advertising from the user’s point of view by providing users with a tool to understand why they are targeted with a particular ad and to infer what information the ad engines possibly have about them. Concretely, we propose to build a browser plugin that collects the ads shown to a user and provides her with analytics about these ads. Our tool relies on an innovative collaborative approach to infer what information the ad engine may have.