Abstract: Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for extending publisher inventory. In one aspect, a method includes receiving distribution agreement data specifying a first number of impressions that a first publisher has agreed to allocate to a content item provider during a specified period. A determination is made that an estimated total number of impressions that the first publisher will allocate to the content item provider over the specified period will be less than the first number of impressions. A third number of additional impressions are obtained from one or more second publishers that differ from the first publisher. At least a portion of the third number of additional impressions is allocated to the content item provider to meet the first number of impressions specified by the distribution agreement data.