Abstract: Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for tracking and utilizing offline conversion data are disclosed. In one aspect, a method includes receiving, from a user device, interaction data specifying the occurrence of a particular user interaction with the content item. An updated path for a landing page of the content item is provided to the user device. The updated path includes an interaction identifier that represents the particular user interaction. Offline conversion data are received from a content sponsor associated with the content item. The offline conversion is attributed to the particular user interaction based on the interaction identifier being included in the offline conversion data. An adjusted bid is determined based on the offline conversion data and a bid associated with the content item. The adjusted bid is submitted to a content item selection process.