This book provides a theory, a formal language, and a practical methodology for the specification, use, and reuse of problem-solving methods. The framework developed by the author characterizes knowledge-based systems as a particular type of software architecture where the applications are developed by integrating generic task specifications, problem solving methods, and domain models: this approach turns knowledge engineering into a software engineering discipline. All in all, this work, as an applicable theory of knowledge engineering, consolidates research work done during several decades. The present popularity of Internet-based services will provide unprecedented opportunities for deploying and sharing knowledge-based services and anybody wanting to participate in this area can learn from this book what knowledge engineering is about.