Many of the problems with native XML databases relate to query performance and subsequently, it can be difficult to convince traditional database users of the benefits of using semi- or unstructured databases. In particular, the ongoing development of the XQuery language requires that performance related issues are resolved. Presently, there still lacks an index structure providing efficient support for both navigational and structural queries and the traditional data-centric and content queries. This thesis presents a new extended index structure based on the preorder traversal rank and the level (or depth) rank of each node in a document tree. The extended index fully supports the navigation of all XPath axes while efficiently supporting data-centric queries. The ability to start path traversals from arbitrary nodes in a document tree also enables the extended index to support the evaluation of path traversals embedded in XQuery expressions. Furthermore, an encoding technique for this extended index structure is presented, whereby properties of a level ranking may be exploited to provide efficient and optimised path traversals and in certain cases, optimal solutions to path traversals.