1. A method performed by data processing apparatus, the method comprising: identifying an entity that is referenced by a received search query; identifying a plurality of content items that have been identified as relevant to the entity; for each of the identified content items, identifying: a content category to which the content item has been classified; and a rank score for the content item, the rank score being indicative of the relevance of the content item to the entity; selecting a set of knowledge modules to be presented in a knowledge panel for the entity, the knowledge panel being a portion of a search results page in which the set of knowledge modules are presented, the set of knowledge modules including at least two different knowledge module types, the selection being based, at least in part on, the rank scores and identified content categories for the content items; and providing, for a user device, data that cause the user device to present the set of knowledge modules in the knowledge panel, the knowledge panel being presented with the search results page for the received search query, and each knowledge module being presented in a respective knowledge panel location based, at least in part, on the knowledge module types, each knowledge module presenting one or more of the identified content items.