In this thesis, we investigate the different proposed ways of combining object oriented and relational database systems (such combined systems are commonly called object-relational systems). This thesis is based on ideas presented in various papers about object and object relational databases. In this work, a discussion of standards such as ANSI's SQL3 (to be released) and ODMG-93 is given. In particular, the "Class = Relation" and "Class = Domain" approaches to object-relational systems are investigated. Arguments supporting the proposition that the latter approach is the correct approach are presented.