Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for content distribution. In one aspect, a method includes obtaining reservation data specifying a reservation period and a number of impression reservations for the reservation period; receiving, during the reservation period, content item request data specifying that a resource was requested; identifying candidate content item providers; determining an expected revenue that will be obtained by selecting one of the candidate content item providers to provide a content item; providing, to a content item exchange, auction data including a reserve price; receiving positive response data specifying that the content item exchange will provide an exchange content item for a price that meets the reserve price; and in response to receiving the positive response data, selecting the exchange content item as the content item to be provided in response to receiving the content item request data.