The investigation process consists of data collection, analysis and report writing. Insufficient data collection affects the quality of the investigation report. The site of the accident investigation is immediately after the accident, and there are various levels and kinds of risks. This chapter discusses the safety of investigators on accident sites, protection and collection of evidence, and assessment of the evidence. A generic or static risk assessment will be conducted prior to the on-site investigation by an investigation organization, and dynamic risk assessment needs to be conducted by investigators at the accident scene during on-site investigation. The risk assessment sheet can help investigators to assess risks during onsite investigation. In some countries, legal systems where the criminal investigation takes precedence over the safety investigation allow judicial authorities to have responsibility for setting the cordons. Complete summaries of the interview are required and whether further investigation is needed is decided.