The EXODUS database toolkit, and in particular the E persistent programming language, have been used in two substantial database system implementation efforts by the authors, the Ariel database rule system and the Triton nested relation DBMS. An important advantage of using a persistent programming language for database system implementation is that it is easy to implement special-purpose persistent objects used by the DBMS such as catalogs, rule indexes, and nested relational structures. Support for transactions built into a persistent programming language greatly reduces the effort required to implement a database system. A disadvantage observed is that it is not possible to map the type system of the DBMS to the type system of the underlying programming language while still retaining good performance for ad hoc queries. Also, software engineering difficulties arise when a persistent language makes a distinction between database types and main-memory types.