The purpose of this study was to evaluate the effectiveness of the Olweus Bullying Prevention Program (OBPP) in reducing specific forms of bullying--verbal bullying, physical bullying, and indirect/relational bullying, as well as cyberbullying and bullying using words or gestures with a sexual meaning. This large-scale longitudinal study, which involved more than 30,000 students in grades 3-11 from 95 schools in central and western Pennsylvania over the course of 3 years, employed a quasi-experimental extended age-cohort design to examine self-reports of being bullied, as well as bullying others. Findings revealed that the OBPP was successful in reducing all forms of being bullied and bullying others. Analyses by grade groupings (grades 3-5, 6-8, and 9-11) revealed that, with only a few exceptions, there were significant program effects for all forms of bullying for all grade groupings. For most analyses, program effects were stronger the longer the program was in place. Most analyses indicated similar and substantial effects for both boys and girls, but a number of program by gender interactions were observed. Program effects for Black and White students were similar for most forms of being bullied and bullying others. Although Hispanic students showed results that paralleled the development for Black and White students for particular grade groups and variables, they were overall somewhat weaker. The study provided strong support for the effectiveness of the OBPP among students in elementary, middle, and early high school grades. Program effects were broad, substantial, and largely consistent, covering all forms of bullying--verbal, physical, indirect, bullying through sexual words and gestures, and, with somewhat weaker effects, cyberbullying--both with regard to being bullied and bullying others. Strengths and limitations of the study, as well as future research directions, are discussed.