The term risk has experienced a dramatic growth in usage over the last few years. This is mainly due to the growing concern about the future of mankind, but also to a greater awareness of the limited nature of our resources. People want to know what can go wrong. In the beginning terms such as risk, disaster, indeterminacy and uncertainty are discussed in detail in this book. Furthermore disasters and risks are categorized into natural, technical, health and social risks, and then major disasters in these categories are listed. Risk parameters are introduced to allow the objective description of risks. The explanation of risk parameters (such as mortalities, fatal accident rates, the family of F-N curves and the concept of lost life years) leads to the quality of life parameters as measurements of risk. In general this parameter permits the comparison of the efficiency of all safety measures over the entire human society. However, limitations of this concept are also disclosed. The book clearly shows the interdependence of risk measures. Safety and risks cannot be discussed only by looking at specific problems, since increasing safety in individual fields might lead to a decrease of safety over the entire society.