Book excerpt: Uptake and implementation of WHO’s Mental Health Gap Action Programme Humanitarian Intervention Guide (mhGAP-HIG) has been limited in many settings despite interest and effort from humanitarian actors. According to global assessments, international agencies report a considerable need for further capacity development to implement the tool within their humanitarian health activities. In response, the mhGAP-HIG capacity-building project was initiated in 2018. This report partially assesses the impact of the mhGAP-HIG capacity-building project on individual programmes and at the organizational level through case studies that describe the work of agencies in four complex settings. In reviewing these four examples, the report also identifies key factors that support success in implementation, as well as challenges and lessons learned, which may be useful for informing future implementation of mhGAP-HIG by humanitarian actors working in emergency settings.