  The Virtual Observatory Registry is a distributed directory of information
systems and other resources relevant to astronomy. To make it useful,
facilities to query that directory must be provided to humans and machines
alike. This article reviews the development and status of such facilities, also
considering the lessons learnt from about a decade of experience with Registry
interfaces. After a brief outline of the history of the standards development,
it describes the use of Registry interfaces in some popular clients as well as
dedicated UIs for interrogating the Registry. It continues with a thorough
discussion of the design of the two most recent Registry interface standards,
RegTAP on the one hand and a full-text-based interface on the other hand. The
article finally lays out some of the less obvious conventions that emerged in
the interaction between providers of registry records and Registry users as
well as remaining challenges and current developments.
