Bullying remains a significant social problem that is associated with social and mental health problems for children and youth involved. Continued attention from research is therefore critical to improve interventions that will reduce and prevent bullying among peers. Importantly, most acts of bullying are witnessed by others and these bystanders can play a critical role in stopping and preventing bullying by intervening using prosocial strategies that facilitate peaceful conflict resolution and positive outcomes for all involved. Yet, defending may be costly and most bystanders remain passive observers of face-to-face and online bullying, taking no action to defend. Also, rather than using prosocial defending strategies, some bystanders use aggressive defending by targeting those who perpetrate the bullying, which increases the risk of further aggression. Guided by theory and research on youth and adult prosocial behaviour (e.g., Batson et al., 1987; Eisenberg et al., 1989, 2010; Hoffman, 2001; Decety & Lamm, 2009; Singer & Klimecki, 2014; Stevens & Taber, 2021), the overarching aim of this research was to identify the unique role of compassion, separate from empathic distress, empathic anger, and perceived social costs, in youth's defending intentions after witnessing peer bullying. [...]