This research handbook fills the need to investigate and adapt mediation in response to three major changes in the international system. According to Wilkenfeld, Beardsley & Quinn, these changes are the decline in interstate conflicts, increase in gray zone conflicts, and transnational threats to human security. In line with such changes, the book is organised in four thematic sections. The chapters in the first section highlight both empirically and conceptually the changes in crisis, mediation, and their complexity over time. The second part provides new case studies as well as using improved data and measurements to examine the occurrence and outcomes of interstate mediation. In the third section, the need to focus on domestic aspects of mediation, both due to the increase in intrastate conflicts, as well as the involvement of domestic mediators, is demonstrated. The final section of the book looks at a variety of emerging contemporary security challenges, ranging from cyber threats to refugees and climate change. These emerging issues are not the only challenges for future mediation where, for example, the underrepresentation of female mediators remains an issue. While mediation is not the only way to address such issues, the research handbook argues, and provides convincing evidence, that mediation remains an important tool in responding to the increasing complexity of contemporary crisis and conflicts. This edited volume contains an impressive range of topics and approaches and is skilfully bound together by the three main themes about the changing international system. The handbook concludes with policy recommendations, making the book interesting and relevant not only to students and scholars of mediation, but to practitioners as well.