This document is for humanitarian health actors working at national and sub-national level in countries facing humanitarian emergencies. It applies to Health Cluster partners, including governmental and non-governmental health service providers. Based on the IASC Guidelines on Mental Health and Psychosocial Support in Emergency Settings (IASC, 2007), it gives an overview of essential knowledge that humanitarian health actors should have about mental health and psychosocial support (MHPSS) in humanitarian emergencies. This document by the IASC Reference Group for Mental Health and Psychosocial Support was developed in consultation with the IASC Global Health Cluster.