Search and discovery tools for astronomical on-line resources and services

A growing number of astronomical resources and data or information services are made available through the Internet. However valuable information is frequently hidden in a deluge of non-pertinent or non up-to-date documents. At a rst level, compilations of astronomical resources provide help for selecting relevant sites. Combining yellow-page services and meta-databases of active pointers may be an ecient solution to the data retrieval problem. Responses generated by submission of queries to a set of heterogeneous resources are dicult to merge or cross-match, because dierent data providers generally use dierent data formats: new endeavors are under way to tackle this problem. We review the technical challenges involved in trying to provide general search and discovery tools, and to integrate them through upper level interfaces.