Information systems have evolved from simple file processing systems to powerful database systems. Relational Databases put the layout and form into the unchanging metadata and gave us “record once” systems. Unfortunately, the “real world” upon which databases are modelled constantly changes. These changes may affect the schema for a variety of reasons. Different formalisms have been developed to deal with schema changes aiming to preserve information capacity and semantic correctness. Schematic conflicts occur which must be resolved. Data integration or conversion remains a partially resolved issue. This book introduces a three-level architecture for RDMS with an interface, named the mesodata layer, between data and metadata for complex domains. The mesodata layer provides complex structures in which to store domain values and their inter-relationships as well as supplying the “intelligence” required to manipulate them. The domain structures enable different orderings for enhanced querying and information retrieval. This approach should interest database modellers who manage changes to a database, such as structural change, semantic change and constraint change.