This book is based on columns and tutorials published in the Bulletin of the European Association for Theoretical Computer Science (EATCS) during the period 2000-2003. It presents many of the most active current research lines in theoretical computer science. The material appears in two volumes, "Algorithms and Complexity" and "Formal Models and Semantics", reflecting the traditional division of the field.The list of contributors includes many of the well-known researchers in theoretical computer science. Most of the articles are reader-friendly and do not presuppose much knowledge of the area in question. Therefore, the book constitutes very suitable supplementary reading material for various courses and seminars in computer science.