Appropriate guidelines for measuring haemoglobin and defining anaemia are crucial for both clinical and public health medicine but require consideration of a range of complexities across different populations. The objective of this guideline is to provide updated, clear, evidence-informed normative statements on the use of haemoglobin concentrations to assess anaemia and on the best approaches in its measurement in individuals and populations. The purpose of the guideline is to improve the diagnosis of anaemia, grounded in gender, equity and human rights approaches, with the aim of leaving no one behind, thereby informing the development of nutrition and health policies. The normative statements in this guideline are intended for a wide audience, including policy-makers, their expert advisers, and technical and programme staff at ministries and organizations involved in the design, implementation, and scaling-up of actions for addressing anaemia. Other end-users of the guideline include nongovernmental and other organizations and professional societies involved in the planning and management of anaemia actions, as well as health professionals including clinicians, managers of nutrition and health programmes, national blood services, and public health policy-makers in all settings.