An assessment of the growth of private security agencies over the last 150 years considers their encroachment into areas formerly reserved for public police. Evidence supports the thesis that private security has grown due to the inability of police to adapt to major social or technological change economically. The analysis of the physical security offenses most often investigated by private security personnel focuses on trespass, burglary, vandalism, and larceny. The personal security offenses commonly probed by private investigators involve crimes against persons, including homicide, kidnapping, and adultery. Some offenses investigated by private security involve fraud or misrepresentation. The legal authority of private security is compared to that of government officials, followed by descriptions of five common investigative techniques used by private investigators: record checks, interrogations, surveillance, polygraphs, and drug testing. The powers of arrest and search and seizure in private security are compared to those held by government agents. A review of civil and criminal liabilities commonly faced by private security personnel include false arrest, assault, emotional distress, defamation, and privacy invasion. The book concludes with the outline of a strategy for preventing shoplifting and employee theft. 22 references, subject index.