This study explores the impact of bullying on the mental health of students who witness it. A representative sample of 2,002 students aged 12 to 16 years attending 14 schools in the United Kingdom were surveyed using a questionnaire that included measures of bullying at school, substance abuse, and mental health risk. The results suggest that observing bullying at school predicted risks to mental health over and above that predicted for those students who were directly involved in bullying behavior as either a perpetrator or a victim. Observing others was also found to predict higher risk irrespective of whether students were or were not victims themselves. The results are discussed with reference to past research on bystander and witness behavior. (Contains 4 tables.)