Methods, systems, and apparatus, including computer programs encoded on a computer-readable storage medium, and including a method for providing content. The method comprises receiving a first login request from a first device used by a user, the request being associated with a first anonymous identifier associated with the first device, and determining a user tag for the user, that does not include any personally identifiable information associated with the user. The method further comprises receiving a second login request from a second different device used by the user, the request being associated with a second different anonymous identifier associated with the second different device, and storing an association between the user tag, the first anonymous identifier and the second different anonymous identifier. The method further comprises receiving a request for content from either the first or second different device and providing content in response to the request using the association.