The CGIAR Research Program (CRP) on Aquatic Agricultural Systems (AAS) led by WorldFish aims to lift millions of people out of poverty and promote positive, transformative change in aquatic agricultural development. The AAS program recognizes that gender and power inequalities between men and women, which are reinforced at all levels of society, are key factors perpetuating poverty. One of the AAS themes aims to engage men as an integral part of its gender-transformative approach to the questioning and fundamental altering of unequal power relations and structures. These goals require the creation of spaces for critical reflection and action focused on harmful masculinities and their effects on the lives of women, girls, and men themselves - in combination with other approaches. However, thematically relevant resources (i.e. those that provide a conceptual and programmatic approach to such reflection with men and boys) have been lacking. To help fill this gap, this manual was developed by Promundo, based on experiences training in AAS country "hubs" (i.e. geographic locations providing a focus for innovation, learning, and impact through innovation research) in 2014 and informed by other resources related to the engagement of men and boys.