Safety investigation is here described in details following the basic five areas (People, Environment, Equipment, Processes and Procedures, Organization and External Influences). In this module will be studied a casualty investigation code, the proper use of equipment in accident investigation, the most suitable methodology of investigation, how to collect data and evidences, how to inspect casualty sites and to make interview to witnesses, how to conduct safety analysis and to make a correct reconstruction of the casualty events and their linked conditions, how to fill in investigation reports.