The term ”Metadata” is often used by information professions to refer to character the resources contained in a digital library. This paper adopts the broadest meaning of ”Metadata”-Data about Data, and applies metadata to facilitate the inter-operation among digital library systems. It first defines digital library interoperability, its related issues and approaches. Then, citing as an instance the Stanford Digital Library Project to describe the roles that metadata can play in handling digital library interoperability. This paper also describes how National Chiao Tung University has tried to solve the problem of handling digital library interoperability by proposing the Metadata Modeling Language (MML), which is a language to define the metadata of digital-library contents and services, and define the translation of two kinds of metadata. Finally, a virtual union catalog system based on MML, called VUCS@NCTU, is implemented to vindicate the feasibility of MML.