Abstract: Data-intensive Web sites have created a new form of knowledge base, as richly structured bodies of data. Several novel systems for creating data-intensive Web sites support declarative specification of a site's structure and content (i.e., the pages, the data available in each page, and the links between pages). Declarative systems provide a platform on which AI techniques can be developed that further simplify the tasks of constructing and maintaining Web sites. This paper addresses the problem of specifying and verifying integrity constraints on a Web site's structure. We describe a language that can capture many practical constraints and an accompanying sound and complete verification algorithm. The algorithm has the important property that if the constraints are violated, it proposes fixes to either the constraints or to the site definition. Finally, we establish tight bounds on the complexity of the verification problem we consider.