In this paper we have classified the major combined simulation languages into six approaches. The approaches are useful for understanding the historical development and current direction of combined simulation modeling. We have also characterized the functional capabilities of these languages. We have further traced the relationships of the languages over time. The relationships show which languages have evolved from other languages. Finally, from the context of the classification and relationships, many of the unresolved research issues are presented. While we have reviewed all of the languages, particular emphasis has been placed on the object-oriented languages since they have not been thoroughly reviewed to date.