Objective: Despite the proliferation of bystander approaches to prevent aggression among youth, theoretical models of violence-related bystander decision making are underdeveloped, particularly among adolescents. The purpose of this research was to examine the utility of 2 theories, the Situational Model of Bystander behavior (SMB) and the Theory of Planned Behavior (TPB), for identifying mechanisms underlying adolescent bystander behavior in the context of bullying and teen dating violence (TDV). Method: Data were collected via face to face (local) and online (national) focus groups with 113 U.S. adolescents aged 14–18 and were subsequently analyzed using deductive and inductive coding methods. Results: Youth endorsed beliefs consistent with both the SMB and TPB and with additional constructs not captured by either theory. Adolescents reported a higher proportion of barriers relative to facilitators to taking action, with perceptions of peer norms and social consequences foremost among their concerns. Many influences on bystander behavior were similar across TDV and bullying. Implications: Findings are organized into the proposed Situational-Cognitive Model of Adolescent Bystander Behavior, which synthesizes the SMB and TPB, and supports the tailoring of bystander interventions. For teens, intervening is a decision about whether and how to navigate potential social consequences of taking action that unfold over time; intervention approaches must assess and acknowledge these concerns. (PsycINFO Database Record (c) 2017 APA, all rights reserved)