This book introduces some of the most important results in theoretical computer science. The "gems" are central problems and their solutions from the areas of computability, logic, circuit theory, and complexity. The text presents complete proofs in understandable form, as well as previously open problems that have found a (perhaps unexpected) solution, complex proofs from bottom drawers, probabilistic constructions, and much, much more. With over 240 intriguing exercises (elegant solutions for which are supplied), the text also challenges the reader to do some active work.

While I was visiting Boston University during the 1996-97 academic year, I noticed a small book, written in German, on a shelf in Steve Homer's office. Curious, I borrowed it for my train ride home and began reading one of the chapters. I liked the style and format of the book so much that over the course of the next few months I frequently found myself reaching for it and working through one chapter or another. This was my introduction to Peden der Theoretischen Informatik. A few of my colleagues had also seen the book. They also found it inter- esting, but most of them did not read German well enough to read more than small portions of it enjoyably. I hope that the English version will rectify this situation, and that many will enjoy (and learn from) the English version as much as I enjoyed the German version. The front matter of this book says that it has been "translated, revised, and expanded." I should perhaps say a few words about each of these tasks. In translating the book, Ihave tried as much as possible to retain the feel of the original, which is somewhat less formal and impersonal than a typical text book yet relatively concise. I certainly hope that the "pleasure of the pursuit of understanding" has not gotten lost in the translation.