An expanded Theory of Planned Behaviour (TPB; Ajzen, "Organizational Behavior and Human Decision Processes" 50:179-211, 1991, Ajzen, "Journal of Applied Social Psychology" 32:665-683, 2002) which incorporates affective, moral, and personal belief variables with Ajzen's original social, behavioural, and cognitive factors has not yet been applied to bullying and bully-victimization in junior high school students. This study addresses this gap by applying this expanded version of TPB with a sample of 342 junior high school students (M[subscript age] = 12.27, 207 girls and 135 boys) from seven schools from a large Western Canadian municipality. Overall, 11.08% of participants were classified as students who bully others ("bullies") and 13.21% as students who bully others and are also victimized themselves ("bully-victims"), with boys reporting higher levels than girls. Structural equation modelling was used to test a TPB-informed model to explain the relations between psychological adjustment, self-concept, attitude and beliefs about aggression, perceived control, intention, and bullying behaviour. The final models accounted for 40% of the variance in bullying and 34% of the variance in bully-victimization, although the models differed. Both bullying behaviour and bully-victimization were predicted by less support for the victim, normative beliefs supporting aggression/bullying, and less well-developed moral reasoning about aggression, which was, in turn, predicted by higher levels of sensation seeking and hyperactivity (both) and depression and lower ego strength (bully-victim only). For bullying behaviour, there was a direct effect of internalizing behaviour on control. In contrast, attitude and beliefs about aggression and anger control mediated the relations between internalizing/externalizing behaviour and bully-victimization. Implications of this work are discussed, including anti-bullying attitude and belief initiatives.