Preface

The chapters in this volume were designed to cover the broad range of existing research and to suggest productive lines of development. However, even the relatively short production lags in these volumes imply that a number of new and exciting works are only hinted at in the chapters. In short, there is much more work to be done as this field unfolds. A variety of factors went into the selection of authors of these chapters. Clearly, a fundamental requirement was that the authors had to be leaders in the intellectual development of the various topics. But, beyond that, authors were selected because they had a point of view, one designed to provoke thought and new work. The ideas put forward in the chapter are likely to be challenged in further work. The idea is not to write the final word on any of these topics, because each is the source of lively current debate. The idea instead is to provide an intermediate assessment of dynamic research areas to push the research further.