This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book describes individual and organizational accidents. There are two kinds of accidents: those that happen to individuals and those that happen to organizations. The book begins the finer-grained analysis by examining the various cracks in the system' revealed by case studies of three organizational accidents. Organizational accidents have multiple causes involving many people operating at different levels of their respective companies. The book shows the relationship between the three elements: hazards, defences and losses. All organizational accidents entail the breaching of the barriers and safeguards that separate damaging and injurious hazards from vulnerable people or assets collectively termed losses'. The book presents three case studies that illustrate some of the different ways that active failures and latent conditions can bring about the partial or total penetration of a system's defences.