Examples of undercover activities used against prostitution, pornography, selling and buying stolen goods, robbery, burglary, assault, organized crime, white-collar crime, and corruption are presented. In such tactics, police are disguised as typical victims to act as 'bait' for potential offenders or as underworld characters who construct scenarios that provide the opportunity and sometimes the encouragement for lawbreaking. While many of the undercover tactics border on entrapment, this is difficult to prove in court. The goals of undercover activity are to increase arrests and presumably deter the commission of the crimes targeted; however, there is no evidence that undercover work reduces the aggregate rate of the offenses involved. The undercover activities, particularly those that construct the influences that stimulate crime, can be criticized from the perspective of a number of criminological theories. By constructing the conditions that cause crime rather than prevent it, the police fail to challenge the conditions that breed crime. Further, by enticing persons, in many cases first offenders, into offenses, the labeling theory holds that a criminal identity is being established and criminal patterns are likely to be reinforced rather than diminished. Undercover practices are most acceptable when the offense is a felony, when there is a complainant other than the State, and when the decoy is the victim. Such practices are also most acceptable when used in public, when the appropriate evidence cannot be obtained in more conventional ways, and when police have a suspect and sufficient information to otherwise justify wiretap or search warrants. Finally, they are most acceptable when some verification of the undercover operative's account is possible, and with a more observational and passive police role. Seventy notes and references are provided.