In this paper we have classified the major combined simulation languag
es into six approaches. The approaches are useful for understanding th
e historical development and current direction of combined simulation
modeling. We have also characterized the functional capabilities of th
ese languages. We have further traced the relationships of the languag
es 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. W
hile we have reviewed all of the languages, particular emphasis has be
en placed on the object-oriented languages since they have not been th
oroughly reviewed to date. (C) 1997 Elsevier Science Ltd.