This country implementation guide outlines current evidence, knowledge and best practices relating to the health and health challenges of refugees and migrants in immigration detention, as well as alternatives to detention. It highlights key principles and international commitments, summarizes the current status and health challenges and provides practical considerations for addressing the health challenges of refugees and migrants in immigration detention, as well as the implementation of alternatives to detention. Specific areas for intervention include providing comprehensive training for staff, ensuring safeguards, providing psychological support and providing tools to prevent the spread of communicable diseases. The guide also promotes engagement-based alternatives to detention. While the main intended audience is policy-makers across sectors at local, national and regional levels, the guide is also of value for health planners, relevant ministries, international organizations, management of immigration detention facilities and their staff, and researchers.