This book provides a comprehensive overview of Human Activity Detection or Recognition (HADR) systems. Detection or recognition of human activities is a prominent research area in the fields of computer vision and artificial intelligence because of its many applications in daily life, including monitoring in public transport areas, health monitoring, anomaly detection in traffic, and smart homes. This book divides different activities according to their criticality, then discusses the various motivations and challenges that are involved in HADR systems. The authors then propose a framework for activity detection or recognition. The book also covers ten key applications of HADR systems and the recent developments for each of them. The authors also propose areas for future research.