A method of retrieving content for inclusion in a plurality of information resources of a content publisher, the method comprising: responsive to receiving, from a client device, a request for first instructions to insert into a header of a first information resource of the content publisher for selecting one of a plurality of content providers, generating, by a data processing system having one or more processors, a session identifier corresponding to the client device, the session identifier for use in selecting content to be served across the information resources; providing, by the data processing system, the session identifier to the plurality of content providers; subsequently receiving, by the data processing system, a request for second instructions to insert into a header of a second information resource of the content publisher for selecting one of the plurality of content providers; determining, by the data processing system, that the second request is from the client device; responsive to determining that the second request is from the client device, obtaining, by the data processing system, respective content selection parameter values for the content providers; selecting, by the data processing system, a content provider from the plurality of content providers using the plurality of content selection parameter values obtained from the plurality of content providers; and providing, by the data processing system, a content retrieval script to the client device to insert into the header of the second information resource, the content retrieval script configured to cause the client device to request the selected content provider to provide a content item for insertion into the second information resource; the method further comprising: providing, by the data processing system, responsive to receiving the first request, a parameter value retrieval script to the client device to insert into the first header of the first information resource, the parameter value retrieval script configured to: permit the corresponding content provider to access state data stored on the client device using the session identifier and to retrieve and cause the client device to obtain a first plurality of content selection parameter values from each content provider of the plurality of content providers, each of the first plurality of content selection parameters used to select a first content item to insert onto the first information resource. responsive to receiving, from a client device, a request for first instructions to insert into a header of a first information resource of the content publisher for selecting one of a plurality of content providers, generating, by a data processing system having one or more processors, a session identifier corresponding to the client device, the session identifier for use in selecting content to be served across the information resources; providing, by the data processing system, the session identifier to the plurality of content providers; subsequently receiving, by the data processing system, a request for second instructions to insert into a header of a second information resource of the content publisher for selecting one of the plurality of content providers; determining, by the data processing system, that the second request is from the client device; responsive to determining that the second request is from the client device, obtaining, by the data processing system, respective content selection parameter values for the content providers; selecting, by the data processing system, a content provider from the plurality of content providers using the plurality of content selection parameter values obtained from the plurality of content providers; and providing, by the data processing system, a content retrieval script to the client device to insert into the header of the second information resource, the content retrieval script configured to cause the client device to request the selected content provider to provide a content item for insertion into the second information resource; the method further comprising: providing, by the data processing system, responsive to receiving the first request, a parameter value retrieval script to the client device to insert into the first header of the first information resource, the parameter value retrieval script configured to: permit the corresponding content provider to access state data stored on the client device using the session identifier and to retrieve and cause the client device to obtain a first plurality of content selection parameter values from each content provider of the plurality of content providers, each of the first plurality of content selection parameters used to select a first content item to insert onto the first information resource. permit the corresponding content provider to access state data stored on the client device using the session identifier and to retrieve and cause the client device to obtain a first plurality of content selection parameter values from each content provider of the plurality of content providers, each of the first plurality of content selection parameters used to select a first content item to insert onto the first information resource.