Bullying is a problem in most, if not all, contemporary schools around the world and is associated with socioemotional, academic, and physical difficulties for not only the victims of bullying, but also the youth who engage in bullying (Beran et al., 2008; Hawker & Boulton, 2000) and bystanders (Janosz et al., 2008; Juvonen et al., 2003). As the actions of bystanders contribute to the deterrence or maintenance of bullying (Salmivalli, 2010), it is critical to understand those factors that lead bystanders' to either protect the victim or facilitate the bullying. To this end, the articles in this special issue provide novel insights into bystanders of bullying, building upon emerging themes in the literature. This introduction summarizes the contributions of each article, focusing on three themes: (a) the process-oriented nature of the decision to defend victims or reinforce the aggressors, (b) the need to differentiate qualitatively different forms of defending, and (c) the role of classroom and societal norms in bystanders' behaviors. This introduction concludes with a call for a next generation of studies that integrates the lines of research presented in this special issue.

欺凌是世界上大多数（如果不是全部）当代学校的一个问题，并且不仅与欺凌受害者而且对参与欺凌的青年人的社会情感、学业和身体困难有关（Beran 等人，2008 年） ；Hawker & Boulton，2000 年）和旁观者（Janosz 等人，2008 年；Juvonen 等人，2003 年）。由于旁观者的行为有助于威慑或维持欺凌行为（Salmivalli，2010），了解导致旁观者保护受害者或助长欺凌行为的因素至关重要。为此，本期特刊中的文章以文献中的新兴主题为基础，为欺凌的旁观者提供了新颖的见解。本介绍总结了每篇文章的贡献，重点关注三个主题：(a) 为受害者辩护或加强攻击者的决定的面向过程的性质，(b) 需要区分质量上不同的辩护形式，以及 (c) 课堂和社会规范在旁观者行为中的作用。本介绍最后呼吁进行下一代研究，整合本特刊中介绍的研究方向。