19. A system configured to respond to a request to create a retargeting set by a content item provider, comprising: a data store storing a retargeting set for a retargeted content item, the retargeting set including retargeting identifiers that each specify a user identifier that was received with interaction data indicating that a pre-specified user interaction which facilitates targeting retargeted content items previously occurred; and a data processing apparatus configured to interact with the data store, the data processing apparatus being further configured to perform operations including: providing a code segment that upon execution by a browser causes the browser to submit 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; 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 for the retargeted content item; 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.
22. The system of claim 19 , wherein the data processing apparatus is further configured to perform operations including: identifying at least one retargeted content item having a targeting keyword that is matched by the search query; determining that the user identifier from the request matches a retargeted identifier for the at least one retargeted content item; determining that the user identifier from the request also matches a particular non-targeted user identifier in a set of non-targeted user identifiers, the set of non-targeted user identifiers specifying one or more user identifiers that must not be matched by the user identifier from the request in order for the at least one retargeted content item to be eligible for presentation; and removing, in response to determining that the user identifier matches the particular non-targeted user identifier, the at least one retargeted content item from the plurality of content items that are eligible for presentation with the search results page.