A major challenge in building the Semantic Web is resolving differences among heterogeneous databases. This article describes one approach for handling semantic data integration problems in hierarchical domains. It also describes a declarative approach for specifying pairwise mappings between a centrally maintained ontology and each local data repository maintained by an autonomous agency. In this context, it outlines a method for specifying the mappings' semantics and encoding them to resolve heterogeneities. It focuses on XML-based applications in which entities in the centrally maintained ontology are hierarchically related to those in the local data repositories.