The purpose of this Report is to provide guidance for investigating radiological incidents that can occur wherever radioactive materials are handled, stored, used or transported, or where radiation generating equipment is operated. Radiological incidents have the potential to adversely impact; the health and safety of workers or members of the public, the environment, operations, and compliance with regulations. This Report provides guidance and practical information for individuals who have the responsibility of performing or overseeing investigations to include a scaled approach such that the extent and rigor of the investigation can be tailored to the severity and complexity of the incident. Guidance is provided on appointing individuals to an incident investigation team including recommendations for the training and qualifications of investigators and the use of consultants and specialists in conducting the investigation. The process of investigation includes a discussion of the initial response to the incident, including the procedures for controlling the incident scene to prevent loss of information, recovering any physical items that may have been removed, and how to gather information related to the incident. Various aspects for conducting the investigation are discussed including the initial team meeting, performance of onsite inspections, interviewing personnel involved in the incident, and collecting physical evidence. Performance of the cause analysis is reviewed including which type of cause analysis to perform. Ideas for the development of a corrective action plan and preparation of the investigation report, including legal considerations, are provided along with suggestions for scheduling, reviewing, tracking and trending the effectiveness of corrective actions. The Report will be useful to all safety personnel, managers who are responsible for operations that involve radiation, and those asked to perform an investig