Bystanders play a crucial role in encouraging or preventing bullying situations and feature prominently in several international antibullying programs (e.g., KiVa). Despite a surge of recent interest in bystanders, relatively little is known about the functional reasons why individuals choose to engage with or ignore bullying incidents. Given the importance of bystanders' influence on bullying, we argue that further consideration needs to be given to the individual costs and benefits of bystanders' intervention. Adolescents in our study (N = 101, M = 15.37 years) read different bullying scenarios and were then asked to respond with how the bystander would react in each scenario while considering and explaining potential personal costs and benefits. We focused on the cognitive reasoning of important factors adolescents may consider when faced with the decision of whether to intervene or not in a bullying situation. Our study provides novel evidence that adolescents engage in quite explicit cost-benefit decisions regarding their decisions of whether or not they would intervene in bullying. The content and structure of these cost-benefit decisions support an adaptive model of bullying behavior and may be helpful in developing more targeted peer-based antibullying programs.