In this thesis we proposed a definition for multivalued dependencies (MVDs) in XML. As in relational databases, MVDs in XML generalize functional dependencies (FDs) and suggest that the XML database can be losslessly decomposed into smaller databases. The main goal of identifying MVDs in XML is to detect the potential redundancy they may cause. We propose an XML Normal Form, XNFM, to address the redundancy issue related to MVDs. XNFM generalizes 4NF and coincides with nested normal form (NNF) for nested schemas. It is an extension of XML normal form (XNF), and thus a more powerful normal form. The normalization algorithm to convert an arbitrary XML document type definition (DTD) into a normalized one, in XNFM, is also introduced. We show that the algorithm outputs a lossless decomposition of the original DTD and the transformations in the algorithm can be implemented by using XQuery, an XML query language developed by the World Wide Web Consortium.As the World Wide Web becomes a major means of disseminating and sharing information, eXtensible Markup Language (XML) is rapidly emerging as a dominant standard for representing and exchanging data in the Internet. Meanwhile, the request and interest in efficiently storing and querying XML repositories has increased.