8. A system, comprising: a data processing apparatus; and a memory storage apparatus in data communication with the data processing apparatus, the memory storage apparatus storing instructions executable by the data processing apparatus and that upon such execution cause the data processing apparatus to perform operations comprising: receiving a content item request that includes context data specifying a user group that will be presented a content item and a resource on which the content item will be presented; in response to receiving the content item request: identifying a given content item that is eligible to be presented in response to the content item request; identifying a template feed from which a template for the given content item is to be selected, the template feed including a set of templates; determining, for each particular template from the set of templates, a contextual performance measure based on (i) a performance of one or more first content items when the particular template has been used to create the one or more first content items for presentation in response to content item requests that included a particular attribute of the context data and (ii) a performance of one or more second content items when the particular template has been used to create the one or more second content items for presentation with the resource; selecting, from the template feed and based on the contextual performance measure of each template in the set of templates, a template for the given content item; populating the selected template with content for the given content item to create a formatted content item; and providing the formatted content item in response to the content item request.
8. A system, comprising: a data processing apparatus; and a memory storage apparatus in data communication with the data processing apparatus, the memory storage apparatus storing instructions executable by the data processing apparatus and that upon such execution cause the data processing apparatus to perform operations comprising: receiving a content item request that includes context data specifying a user group that will be presented a content item and a resource on which the content item will be presented; in response to receiving the content item request: identifying a given content item that is eligible to be presented in response to the content item request; identifying a template feed from which a template for the given content item is to be selected, the template feed including a set of templates; determining, for each particular template from the set of templates, a contextual performance measure based on (i) a performance of one or more first content items when the particular template has been used to create the one or more first content items for presentation in response to content item requests that included a particular attribute of the context data and (ii) a performance of one or more second content items when the particular template has been used to create the one or more second content items for presentation with the resource; selecting, from the template feed and based on the contextual performance measure of each template in the set of templates, a template for the given content item; populating the selected template with content for the given content item to create a formatted content item; and providing the formatted content item in response to the content item request. 14. The system of claim 8 , wherein the operations further comprise: receiving filtering data specifying a proper subset of templates in the set of templates that are eligible to be used for content item requests including a particular set of context data; and filtering the set of templates based on the filtering data and the context data of the received content item request.