The rapid growth of computer and Internet technologies and their acceptance as a viable medium for human services delivery has opened up new possibilities for providing effective prevention programming. This article reviews some of the major online prevention websites and evaluations of their effectiveness. In addition, it reports the struggle to evaluate an interactive multimedia anger management exercise that is part of a teen substance abuse prevention website. The evaluation provided some support that a 30-minute exposure to a web-guided prevention exercise could increase teens' prevention knowledge and that completing the online exercise as supplemental homework reinforces the classroom experience. However, positive changes in other measures of behavior change were not supported. The study pointed to many issues that occur when developing and evaluating online interventions in real-world environments.This background section presents the advantages of online prevention, a brief overview of prevention research and its advantages, and an overview of prevention websites and their evaluation.