System approaches embody the partitioning of problems into smaller inter-related subsystems that will be solved together. Thus, words like hierarchical, dependence, inference, model, and partitioning are frequent throughout this text. Each of the authors deals with the complexity issue in a similar fashion, but the real value in a collected work such as this is in the subtle differences that may lead to synthesized approaches that allow even more progress. The works included in this volume are an outgrowth of the 2nd International Workshop on System Test and Diagnosis held in Alexandria, Virginia in April 1998. The first such workshop was held in Freiburg, Germany, six years earlier. In the current workshop nearly 50 experts from around the world struggled over issues concerning the subject. In this volume, a group of workshop participants was invited to provide a chapter that expanded their workshop presentations and incorporated their workshop interactions. While we have attempted to present the work as one volume and requested some revision to the work, the content of the individual chapters was not edited significantly. Consequently, the reader should see different approaches to solving the same problems and occasional disagreement between authors as to definitions or the importance of factors.