Data collection was done by trained civilians riding on 900 patrol shifts who recorded information on 5,688 police-citizens encounters in 24 police departments in the metropolitan areas of St. Louis, Rochester, N.Y., and Tampa-St. Petersburg, Fla. From these, 742 pertinent encounters were extracted for analysis. Most police contacts with suspects involved misdemeanor offenses; 81.7 percent of the suspects were male, 51.6 percent were white; the predominant age was 19-35 years. Victims were present in less than half of the cases -- 21.3 percent of the victims requested suspect's arrest; 35.4 percent requested informal disposition; 43.3 percent expressed no preference. The decision to take a suspect into custody appeared to be influenced by such features of the situation as the dispositional preferences of victims, the suspect's race and demeanor and the presence of bystanders. Furthermore, the seriousness of the offense increased the chances of arrest. Contrary to much existing literature, males and females were equally likely to be arrested. The research lends some support to both the consensus and conflict models of police behavior. While legal offense seriousness and victim preference motivate arrests, so do suspects' positions of social disadvantage (e.g., being black). Another motivation for arrest decisions is the police need to express authority and appear in control especially before victims and onlookers. The assumption that when victim and suspect know each other, the victim may be less willing to cooperate in the subsequent adjudication may be a deterrent to arrest. Graphs, tabular data, and 41 references are given.