Querying network directories. H. V. Jagadish, Laks V. S. Lakshmanan, Tova Milo, Divesh Srivastava and Dimitra Vista. Hierarchically structured directories have recently proliferated with the growth of the Internet, and are being used to store not only address books and contact information for people, but also personal profiles, network resource information, and network and service policies. These systems provide a means for managing scale and heterogeneity, while allowing for conceptual unity and autonomy across multiple directory servers in the network, in a way far superior to what conventional relational or object-oriented databases offer. Yet, in deployed systems today, much of the data is modeled in an ad hoc manner, and many of the more sophisticated ``queries'' involve navigational access. In this paper, we develop the core of a formal data model for network directories, and propose a sequence of efficiently computable query languages with increasing expressive power. The directory data model can naturally represent rich forms of heterogeneity exhibited in the real world. Answers to queries expressible in our query languages can exhibit the same kinds of heterogeneity. We present external memory algorithms for the evaluation of queries posed in our directory query languages, and prove the efficiency of each algorithm in terms of its I/O complexity. Our data model and query languages share the flexibility and utility of the recent proposals for semi-structured data models, while at the same time effectively addressing the specific needs of network directory applications, which we demonstrate by means of a representative real-life example.