Bystander intervention is a form of helping that occurs when onlookers intercede to provide direct or indirect aid to a victim. When bystanders step in to prevent or reduce harm to others, they act as agents of primary and secondary health prevention. But theory and research suggest the bystander intervention process is complex and multiple social-psychological and situational barriers imperil bystander action. Bystanders are often ill-prepared to intervene when others are at risk for emotional or physical harm. They may not notice that someone needs help due to distraction from self-focus, engagement in social interaction, intoxication, or aspects of the situation like crowding or noise. Due to inadequate knowledge, bystanders may misdiagnose the situation and believe intervention is unnecessary. The negative consequences of nonintervention may be unknown to them such that the situation fails to increase their empathic arousal and motivate their action. Lacking knowledge, they may not recognize the seriousness of the situation and or the potential costs of inaction, and so are insufficiently alarmed. Pluralistic ignorance can arise when multiple uncertain bystanders conceal their concern and hesitate to act, assuming others’ inaction means intervention is inappropriate or unnecessary. When there are multiple witnesses, bystanders may assume their help is unneeded, place intervention responsibility on others, or feel less responsible for helping due to diffusion of responsibility. When the victim is not a member of their in-group, or is assumed at fault for their predicament, they may feel less empathy and a reduced responsibility to help. Or, bystanders may assign responsibility for intervention to the victim’s friends or fellow in-group members, or to those “in charge” of the setting. Even when bystanders realize help is needed and take responsibility for helping, they may not act if they do not know how or lack confidence in their ability to successfully carry out the actions required to help. When they have the skills, they may not help if they perceive the costs of action to outweigh the benefits of action. Audience inhibition arising from group norms supporting inaction and from bystander worry about what others will think about them if they act unnecessarily or ineptly can prevent bystander action by increasing bystanders’ perceived helping costs. Recognition of bystanders as a potentially valuable public health asset has increased interest in promoting bystander intervention. Bystander intervention promotion and communications empower bystander action by combating intervention- and audience-specific barriers to bystander intervention using targeted information, communications, and skills training. Theory and research suggest that effective promotions and communications foster context-specific attitudes, beliefs, norms, and skills such that bystanders: (1) are able to quickly and accurately identify a situation as intervention-appropriate; (2) experience action-motivating arousal (including empathy) in the face of the event; (3) have positive attitudes towards intervention and perceive the benefits of action as outweighing the perceived costs; (4) are empowered to act and feel confident in their ability to effectively intervene (bystander efficacy); and (5) are resistant to evaluation apprehension and norms contraindicating action. Effective bystander intervention promotion draws on social psychology and communications studies, and best practices for health promotion and prevention programs. The application of social marketing and formative and summative program evaluation methods enhance the potential of bystander intervention promotions and communications to empower bystander action.