What is this Book About? At the beginning of the 21st century, computer systems and especially so- ware play an important role in our society. Software is contained in virtually every technical device that we use in everyday life (e.g., cellular phones and cars). Furthermore, computers and their software are used for leisure purposes at home (the Internet and computer games), at the office (e.g., writing letters and order processing), and for more complicated tasks such as controlling steel plants or insuring flight safety. Therefore, the quality of software (e.g., its correctness, re-...