Results of the study indicate that perceived certainty of punishment deters subsequent offending only among those with no or some delinquent peers. Perceived certainty does not have a deterrent effect on offending among those with a high proportion of delinquent peers. It is recommended that future research explore whether there is an interaction between delinquent peers and the perceived severity of punishment. Studies suggest that the perceived certainty of punishment has little or no effect on subsequent offending. Some researchers, however, argue that perceived certainty deters offending among some types of people but not among others. The most recent research on deterrence theory focuses on the factors that condition the effect of formal sanctions on offending. This study contributes to that research by focusing on an important but neglected conditioning variable: association with delinquent peers. The study used longitudinal data from the Youths and Deterrence survey, which involved interviews with high school students from Columbia, SC. Tables, appendix, notes, and references