Background: Bullying threatens the physical and mental well-being of students across a broad range of schools. Tailored interventions based on the Transtheoretical model and delivered over the Internet were designed to reduce participation in each of three roles related to bullying (bully, victim and passive bystander). Methods: Effectiveness trials were completed in 12 middle schools and 13 high schools in the USA. A diverse sample of 1237 middle (6th-8th grade, with 45.1% in the 7th grade; ages 11-14) and 1215 high school (9th-11th grade, with 41.6% in the 9th grade; ages 14-17) students were available for analyses. Results: Analyses showed significant treatment effects for both intervention groups when compared to control for both the middle and high school programs. Conclusions: Given the relative ease of dissemination, these programs could be applied as stand-alone practices or as part of more intensive interventions. (Contains 4 figures and 1 note.)