The history of the private detective industry is reviewed. The latest development in the detective professions is identified as the expansion by some agencies into the field of security services. This trend implies that the distinction between detective agencies and security companies is becoming somewhat blurred, making it dificult to isolate the problems of the two functioning groups. Three broad divisions of the security industry are discussed: companies manufacturing locks and safes, companies producing electronic alarm devices, and companies providing guard services. Emphasizing the manned side of the security industry, discussions center on retail security staff, industrial guards, the armored-car business, and other private security personnel. Problems within the private security industry that are addressed include the possibility of criminals setting up their own firms, misuse of surveillance devices, the proliferation of weapons, and strained relationships between the public and private police. The intrusion of private security forces into society is a growing reality. The real threat in the growth of such services lies in the incidence of poor quality services and inadequate training and instruction of private security personnel. A subject index is appended.