Maintaining the integrity of data is one of the most critical issues involved in designing in database systems. This chapter discusses attribute referential integrity, an expanded concept that allows referential integrity to be specified on any attribute of a relation. It explains the specifications of the new referential integrity constraint on the entity-relationship (E-R) and relational models. The chapter defines extended referential integrity and proposes a notation for the E-R model. Referential integrity in the relational model is defined in various ways. Some definitions assert that to maintain referential integrity, the database must not contain any unmatched foreign key values; others assert that referential integrity is defined only between foreign and primary key relations. In the E-R model, referential integrity is implicitly represented on a relationship type between the participating entity types. Referential integrity is an issue, not only in the context of referencing foreign keys and referenced primary keys.