EXPANDING CONTENT INVENTORY

Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for serving advertisements. In one aspect, a method includes receiving, from a user device, a request for a content item for presentation with a first resource. The first resource is associated with a first content category. A first set of content items that are eligible for presentation with resources associated with the first content category are identified. A determination is made that the user device navigated to the first resource from a second resource. The second resource is associated with a second content category, different than the first content category. A second set of content items that are eligible for presentation with resources associated with the second content category are identified. A content item is selected from among the first and second set of content items. The selected content item is provided to the user device.