A number of definitions to describe major accidents and their specific features exist. In particular, several experts have committed to providing specific and effective definition outlining of low-probability, high-impact events, for which classification is particularly challenging owing to their rarity. These events may result from failures in preassessment, knowledge management, or likelihood evaluation, or they may be simply unpredictable. This chapter reports a brief overview of definitions of such extreme events, from atypical accidents to dragon kings, through the popular metaphor of the black swan. To a certain extent, these different perspectives agree on the fact that conjunctions of “small things” have the potential to result in extreme effects. For this reason, this chapter suggests a twofold approach to be adopted for limitation of such events: Well-known small failures should not be disregarded, and continuous improvement of models and classifications should always be carried out to keep track of the ever-changing industrial environment.