Migrating legacy systems involves replacing (either wholly or partially) existing systems and databases, and complex transformations between old and new data, processes and systems. Correctly performing these activities depends on descriptions of data, and other aspects of the legacy and new systems, and the relationships between them, i.e., metadata. Metadata repositories provide tools for capturing, transforming, storing, and manipulating metadata. They can also store information for managing the migration process itself and for (re)use in other migrations. This paper discusses some of the issues that arise when migrating legacy systems and examines how repository technology can be used to address these issues.