Children who express externalizing behaviors in school are at greater risk of school failure and peer rejection. They are also at greater risk of developing antisocial behaviors, addiction to drugs, mental health problems and delinquency. Many teachers experience difficulties in working with pupils expressing externalizing behaviors. The aim of this thesis is to evaluate a classroom behavior management program called Comet. The main principle of Comet is to get the teacher to use effective strategies when the pupil who is targeted for intervention, and the rest of class, are behaving appropriately and inappropriately. Two versions of the program are evaluated in the thesis: Comet for teachers (Comet-T) and Comet for parents (Comet-P). Three studies are committed to evaluate Comet-T. In the first study Comet-T was compared to an active control group. 100 children (aged 8) were randomized into Comet-T or the control group. At post test and follow-up Comet-T received a better results in reducing externalizing problems, peer problems and teacher behavior management. An analysis of mediators showed that changes in teacher behaviors mediated externalizing behavior. In the second study, children (aged 6 -13) were randomized into two groups. 44 pupils received Comet-T and 42 pupils received a combined intervention consisting of Comet-T at school and Comet-P at home. At post test and follow-up the results show that the combination of Comet-T and Comet-P reduced the externalizing behaviors at home more than Comet-T (only). However, there were no significant differences between the two conditions regarding decrease in externalizing behaviors at school. In the third study Comet-T was compared to a brief version called Comet-TB. At post test Comet-TB had a greater reduction of externalizing behaviors compared to Comet-T. The studies conclude that teachers can use behavior management techniques to decrease externalizing behaviors in the classroom. Furthermore, teachers cannot rely on parent management programs in order to decrease externalizing problems in school. Instead, behavior problems in school need to be solved within school settings. Finally, even a brief program can be effective in order to decrease externalizing behaviors.