The scientific developments at the end of the past millennium were dominated by the huge increase and diversity of disciplines with the common label "computer science". The theoretical foundations of such disciplines have become known as "theoretical computer science". This book highlights some key issues of theoretical computer science as they seem to us now, at the beginning of the new millennium. The text is based on columns and tutorials published in the "Bulletin of the European Association for Theoretical Computer Science" in the period 1995-2000. The columnists themselves selected the material they wanted for this book, and the editors had a chance to update their work. Indeed, much of the material presented here appears in a form quite different from the original. Since the presentation of most of the articles is reader-friendly and does not presuppose much knowledge of the area, the book constitutes suitable supplementary reading material for various courses in computer science.