Building on a notion of keys for XML, we propose a novel indexing scheme for hierarchical data that is based not only on the structure but also the content of the data. The index can be used to check the validity of data with respect to a set of key specifications, as well as for efficiently evaluating queries and updates on key paths. We develop algorithms for the construction and incremental maintenance of the indexing structure, and study the complexity of these algorithms. Finally, we discuss how our indexing techniques can be used for more general queries involving key paths.