The accident investigation process, rooted in the scientific method, relies heavily on the use of evidence. This paper reviews aspects of the investigation process for three large-scale chemical process unit accidents (explosions or fires) with a focus on evidence: its collection, handling, use, and analysis. Real incidents provide challenges to the investigation process; some (of these challenges involve questions surrounding evidence, such as when is evidence collection complete and bow do we know that certain nonphysical evidence (such as witness statements) is reliable? These case studies provide practical examples of the importance of following good engineering practices throughout the life cycle of the investigation with respect to evidence. The objective of an accident investigation. is to determine what happened so that a recurrence of the accident can be prevented. The investigator of a chemical plant accident is confronted with numerous challenges to the fact-finding process because the accident has destroyed or altered significant sources of evidence such as the physical equipment and the process data. ne investigator must often, cope with incomplete or contradictory data. The best strategy for dealing with these difficulties is the scientific method. gather facts from numerous independent sources, develop and test alternative hypotheses, and evaluate potential causal factors using sound engineering and scientific principles the root causes of The ability to ultimately identify a complex chemical plant accident depends heavily on the quality and comprehensiveness of the fact-finding process. (c) 2006 American Institute of Chemical Engineers Process Saf Prog 25: 71-77, 2006.