Cache consistency at web intermediaries is required for scalable web content delivery. In this paper we describe the Web Content Distribution protocol (WCDP), which is an invalidation and update protocol to provide cache consistency for a large number of frequently changing web objects. WCDP supports different levels of consistency: strong, delta, weak, and explicit. It supports atomic invalidates and mutual consistency among objects and handles multiple deployment architectures. WCDP achieves scalability by grouping objects and messages together and by using a hierarchical organization for message delivery. WCDP operates between the origin server, mirror sites, and the participating web intermediaries. It is not, however, targeted for inter-CDN operations.