The architecture of a software system is known to degrade as the system evolves over time due to change upon change, a phenomenon that is termed architectural degeneration. This book concerns with characterisation and diagnosis of architectural degeneration in software systems from the defect quality perspective. In particular, it describes empirical studies on real software systems to address "what do defects indicate about architectural degeneration?". The study results are new and they add to the current knowledge base. This book then proposes a new method to address "how can architectural degeneration be diagnosed from the defect perspective?". This method has been developed in a prototype tool for architectural degeneration diagnosis. Besides, this book describes a comprehensive profile of existing research on measurement, prevention, diagnosis and treatment of architectural degeneration. Overall, this book aims to help increase your knowledge in architectural degeneration and software quality, especially dedicated to researchers, practitioners, and postgraduate students who are interested in software evolution, aging, architecture and quality.