The interrelation of lawsuits is one of the most troublesome, yet least commented upon, areas of the law. The ramifications are great; related lawsuits may be pending concurrently, either brought by the same individual-repetitive litigation--or brought by different parties-reactive litigation. Such lawsuits may occur serially over a period of time. The courts are then faced with problems which have traditionally been discussed in terms of res judicata, bar, merger, or estoppel. It is impossible to cover the whole area or even a sizable part of it in a single article, but it is feasible to examine one facet which certainly warrants consideration: the constitutional involvement in the interrelation of suits.

Before plunging into the topic, it is necessary to consider the terminology used in the discussion. It seems apparent that the courts and commentators are not happy with the terms now being used. There is no clarity or definitude in these terms, especially res judicata and estoppel. In recent years the courts have tended to use the term "preclusion" when referring to a situation in which litigation is foreclosed. To clarify the matter further, in this discussion the term claim preclusion will be used to indicate a situation in which a claim has been decided conclusively and the matter is not open to further litigation. Issue preclusion will be used to describe the situation in which a single issue has been so decided. It is hoped that these terms will help in analyzing the cases and in reaching meaningful conclusions in the area.