This guidance note accompanies the Global Health Cluster’s Standards for Public Health Information Services in Activated Health Clusters and Other Humanitarian Health Coordination Mechanisms. It specifically: provides an overview of the applications of population denominators in the humanitarian health sect or; briefly introduces methods for estimation of population size in emergencies; demonstrates how to appraise and triangulate existing estimates; summarises calculations and parameters required to estimate the size of different vulnerable population groups; describes a systematic approach for estimating health service catchment populations; and suggests a simple method for defining populations in need of humanitarian health services. This guidance note does not contain detailed methods or software/data collection instruments for primary data collection to estimate population denominators. Where appropriate, key methodological references and provided in the bibliography.