The research sought to determine whether parts marking was effective in inhibiting chop shop operations and deterring motor vehicle theft. The participants included investigators from 31 of the country's 32 largest cities, from 6 smaller municipalities, and from 9 State agencies. Information from the 40 jurisdictions that used labels was analyzed separately from the information from jurisdictions that made little or no use of labels. Findings suggested that these anti-theft labels assisted most big-city and State auto theft investigators to arrest car and parts thieves and to prosecute them. Investigators were nearly evenly split about the possible deterrent effects of the labels on auto theft, although some reported that the labels deter some chop shop operators. In addition, anti-theft devices were not considered sufficiently effective to justify exemptions from labeling for cars that manufacturers equip with anti-theft devices. Moreover, most investigators would like the parts marking legislation expanded to include not only all remaining car lines but also noncommercial vehicles and additional parts. Investigators suggested that parts marking might be more effective if auto theft investigators and patrol officers were trained more systematically and often in how to investigate label removal and tampering, if every State made tampering with labels or removing them a felony, and if investigators had access to ultraviolet lights or other detection equipment. Figures, footnotes, appended glossary and table, and 8 references (Author summary modified)