The present study investigated the relationship between cognitive mechanisms applied by people to rationalize and justify harmful acts, and engagement in traditional and cyber bullying among school children. We examined the contribution of Moral Disengagement (MD), Hostile Attribution Bias, and Outcome Expectancies and we further explored the individual contribution of each MD mechanism. Our aim was to identify shared and unique cognitive factors of the two forms of bullying. Three hundred and thirty nine secondary school children completed self-report measures that assessed MD, Hostile Attribution Bias, Outcome Expectancies, and their roles and involvement in traditional and cyber bullying. We found that MD total score positively related to both forms of bullying. Furthermore, traditional bullying positively related to children's moral justification, euphemistic language, displacement of responsibility and outcome expectancies, and negatively associated with hostile attribution bias. Moral justification also related positively to cyberbullying.Cyberbullying and cybervictimization were associated with high levels of traditional bullying and victimization, respectively. The results suggest that MD is a common feature of both traditional and cyber bullying, but it seems that traditional bullying demands a higher level of rationalisation or justification. Moreover, the data suggest that the expectation of positive outcomes from harmful behavior facilitates engagement in traditional bullying. The differential contribution of specific cognitive mechanisms indicates the need for future research to elaborate on the current findings in order to advance theory and inform existing and future antibullying school interventions. Bullying is a subtype of aggression defined as an intentional and repeated aggressive behavior by a group or individual, towards a victim who cannot readily defend himself/herself (Olweus, 1999). Smith and Sharp (1994) describe it as ''a systematic abuse of power'' (p. 2). School bullying is not rare. Various studies in different countries have reported a rate of victimization between 9% -32% and a rate of bullying between 4% -27% (Berger, 2007;Smith et al., 1999). Bullying can be physical (e.g. hitting, kicking), behavioral (e.g. stealing one's lunch,), verbal (e.g. threats, insults), and relational (e.g. exclusion from a group) (Berger). It can be direct or indirect (Underwood, 2000) and it can vary in terms of intensity, duration and motives (Tattum, 1994). In general, boys engage in more physical forms of bullying and girls in more indirect/relational bullying (Rivers & Smith, 1994;Scheitauer, 2002).Bullies and victims suffer from, and are at risk of various psychosocial problems (Card, 2003;Haynie et al., 2001;Hawker & Boulton, 2000;Nansel, Overpeck, Saluja, & Ruan, 2004;Olweus, 1999;Picket et al., 2002).Those who are both bullies and victims, otherwise called aggressive victims, are at higher risk (Duncan, 1999;Wolke, Woods, Bloomfield, & Karstadt, 2000), because they are prone to both ...