The article focuses on a topic of peer bullying, considering experiences of witnesses. A phenomenon of peer bullying is a significant problem from a perspective of school prevention planning. In a majority of theoretical and practical studies, authors describe bullying, focusing on behaviors of perpetrators and victims, omitting a key role of witnesses of bullying. Author of the current article underlines an important role of third persons, who, in their own bold intervention can significantly affect victims defense and therefore can restrict bullying incidents. The article presents chosen typologies of bullying witnesses, underlying a meaning of the role of the witnesses who support victims. Own research is also described, whose aim was to get to know the conditions in which an intervention in a situation of peer bullying can occur. An experiment was conducted using a deepened interview with 23 students aged between 20 and 41 years old. A conditions favoring witnesses to intervene or not to intervene ware described.