Bullying, once considered a "rite of passage" among school children, is increasingly recognized as a serious public health issue. Schools are tasked to both prevent and intervene effectively in bullying; however, the problem does not appear to be decreasing. We know that if teachers see bullying among students and do nothing, then bullying is likely to increase. Teachers are influenced by a number of factors to intervene or not. We discuss these factors, such as teachers' ability to recognize bullying, judgments about how serious a bullying incident is, normative beliefs about bullying, the gender and popularity of the students involved, their own self-efficacy, their empathy, their own stress, the lack of time to effectively intervene and the support of the school leadership. We argue that teacher professional development, including pre-service education, needs to be more nuanced for teachers to take into account these factors to reduce bullying in schools.