Methods and apparatus, including computer program products, for optimization of yield for advertising inventory. A method includes, in a network of interconnected computers, sending a request for a file from a browser residing on a user system to a publisher server, receiving the requested file from the publisher server, the requested file including instructions for the browser to request a site file from a rules server, receiving the site file in the browser, the site file including rules for deciding whether an advertising impression is desired, rules for valuing the advertising impression, and browser executable code that applies and evaluates the rules, executing the browser executable code to evaluate the rules from the site file which determine if one or more signals should be attached to an advertisement request, and selecting an advertisement from a network ad server based on a signal received with the advertisement request from the browser.