12. A non-transitory computer storage medium encoded with a computer program, the program comprising instructions that when executed by data processing apparatus cause the data processing apparatus to perform operations in response to a request to create a retargeting set by a content item provider, comprising: receiving, from a code segment that executing at a browser, interaction data indicating that the code segment was executed, the interaction data including a set identifier for the retargeting set and a user identifier for the user device that caused execution of the code segment; storing, in relation to the set identifier for the retargeting set, any interaction data submitted in response to an execution of the code segment, wherein the user identifier for the user device that caused execution of the code segment is stored as a retargeted identifier corresponding to that user identifier; receiving a request for a content item to be provided with a search results page, the request including data indicative of a search query that was submitted by a user device and a particular user identifier for the user device; identifying a plurality of keyword targeted content items that are eligible for presentation with the search results page, each of the eligible keyword targeted content items being a content item that is eligible for presentation based on the search query matching a targeting keyword for the keyword targeted content item, at least one keyword targeted content item being from the content item provider; determining that one or more retargeted content items are eligible for presentation with the search results page, each of the retargeted content items being a content item that is eligible for presentation with the search results page based on: the search query matching a targeting keyword for the retargeted content item; and the particular user identifier matching the retargeted identifier that is included in the stored retargeting set, each retargeted identifier in the stored retargeting set being a user identifier that was received with interaction data indicating that a pre-specified user interaction which facilitates targeting retargeted content items previously occurred; submitting to an auction the keyword targeted content items that are eligible for presentation and the one or more retargeted content items that are eligible for presentation, wherein at least one of the submitted retargeted content items is different than any eligible keyword targeted content item from the content item provider; selecting, based at least in part on bids that are associated with each of the keyword targeted content items that are eligible for presentation and each of the one or more retargeted content items that are eligible for presentation, a responsive content item to be presented with the search results page; and providing data specifying the responsive content item.
14. The computer storage medium of claim 12 , wherein the program comprises instructions that cause the data processing apparatus to perform operations further comprising: identifying a set of retargeted content items having targeting keywords that are matched by the search query; and removing from the set of retargeted content items at least one retargeted content item having a retargeting set that does not include a retargeted identifier that matches the user identifier from the request.