The analysis rested on comparative data from observations of 100 staged bicycle thefts in and around The Hague, Netherlands and from interviews with 729 randomly-selected residents of Amsterdam. In the staged thefts, a researcher used a large bolt cutter and ostentatiously broke the chain which was securing a racing bicycle. In the interviews, the respondents viewed a photograph of a person using a bolt cutter to steal a bicycle and answered questions regarding their probable behavior if they saw an actual crime of this nature. Whereas most of the persons interviewed said that they would call the police or address the thief, only 5 percent of the bystanders alerted the police and only 2 percent addressed the purported thief. Whereas older females in the interviews were the most likely to contact the police, the elder and/or female onlookers did not show a high tendency to warn the police. Male onlookers were much more likely than females to intervene personally, only because of the high percentage of interventions by males who were accompanied by their wives or female companions. People's opinions about the desirability of interventions in crime are rather poor predictors of actual interventions because real situations are highly variable and seldom as clear as a photograph. Situational aspects of a crime affect the decision to intervene. In one-fourth of the staged incidents in which police officers were nearby, at least one onlooker reported the theft, indicating that foot patrols may be more effective than car patrols in gaining public support for controlling crime. Nevertheless, the anonymous social environments of cities appear to be conducive to crime. Data tables and a list of 15 references are provided.