A method and system for mapping a tag in a markup language (ML) document to a class using namespaces is described. A parser in a computing system analyzes the tag in the ML document. A definition file location attribute related to the tag is referenced in the ML document. A definition file associated with definition file location attribute is retrieved. A namespace related to the tag is referenced within the definition file to determine the class associated with the tag. The class is located in an assembly such that the tag is mapped to the class.