Data were collected through 100 interviews conducted with officers in two large cities in the western U.S. The 12 months of field work included 11 weeks of participant observation carried out in skid row districts. The article aims to clarify conceptually the mandate and the practice of keeping the peace. Structural determinants are defined here as the typical situations perceived by policemen as demanding action without arrest. The description of peace keeping in skid row districts emphasizes those aspects of it that constitute practical skills, as defined by the policemen themselves. This discussion is confined to what police patrolmen consider to be the reality of their work circumstances, what they do, and what they feel they must do to do a good job. 28 notes