David Reiley speaks about field experiments in search engines related to the externalities that advertisements may have among them. His main focus is to show how customers react when they are presented with a wide range of ads, listed together in the same query. Reiley explains the different methods by which companies place their advertisement on the web, and describes the main objectives they seek when doing so. He also explains the process and techniques advertising companies use to maximize the impact on their users. He shows revealing conclusions about this experiment, and finally, comments on the results he obtained from this scientific approach on technological marketing and sales.