There has been a saying that “If you think accident preventionAccident prevention is expensive, try an accident”. Some enterprises have learned this lesson the hard way. Undoubtedly, investigating accidents and learning their lessons are crucial for preventing future analogous accidents. This lesson explores the importance of learning lessons from past accidents. Then, the lesson presents and discusses the critical implications of four notable accidents across different countries. A core focus is placed on the importance of root causesRoot cause, which are the most essential outcomes derived from accident investigationAccident investigation. The lesson further outlines the accident investigation process and compares two types of investigation, i.e., enterprise internal investigation and external investigation. After that, two commonly used investigation modelling techniques based on sequential modelsSequential model are introduced. Further, the lesson presents the integration principle between accident investigationAccident investigation and QRAQuantitative Risk Assessment (QRA) so as to enhance the understanding of the accident scenario and establish a comprehensive risk picture. Lastly, the challengesChallenges of lessons learnedLessons learned from accidents are addressed. Some reputable accident investigationAccident investigation organizations are also introduced for reference. This lesson prepares readers to realize that accident lessons are not about blame but learning, ensuring errors of the past pave the way for a safer future.