Behavioral retargeting consists of displaying online advertisements that are personalized according to each user’s browsing history. At this point, the selection of the products to display in the banner needs to be fast and accurate. At Criteo, we built a recommender system which is able to choose a dozen of relevant products from over two billion products in a few milliseconds. In this talk, we will expose the problems we faced whilst building this system and how we solved them thanks to a mix of online and offline computations.