The aim of the present study was to focus on how students articulate and discuss what factors influence students' decisions to defend or not defend victims when witnessing bullying. In this unique qualitative cross-collaborative study, where two research teams collected interviews from two cultural contexts, eighty-nine students with an age-range from 9 to 14 years old participated. Participants included 43 Swedish students and 46 US students (50 girls, 39 boys). The interviews were analysed through a collaborative qualitative analysis aimed at constructing shared concepts of our data as a whole. The results revealed five broad factors among the students when they reasoned about how they act as a bystander in bullying situations: (a) informed awareness, (b) bystander expectations, (c) personal feelings, (d) behavioural seriousness, and (e) sense of responsibility. The results indicated that each of these considerations could make the students more or less likely to defend as well as to defend in a certain way. According to these five broad factors, students seemed to adjust their bystander acts, which suggests that students' bystander acts vary depending on situational factors that influence bystanders' interpretations of bullying and decision-making about how to respond to observed bullying.