Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for providing asynchronous and synchronous links to resources. According to one example implementation, a method includes receiving a request for a resource, identifying resources to be referenced by the requested resource, and identifying one or more of the referenced resources that are associated with client-side click tracking, and one or more of the referenced resources that are associated with server-side click tracking. The method also includes providing the requested resource. The provided resource includes one or more client-side click tracking links to the referenced resources that are associated with client-side click tracking, and one or more server-side click tracking links to the referenced resources that are associated with server-side click tracking.