The fourth edition of the Handbook on the Least Developed Country Category provides comprehensive information on the least developed country (LDC) category, including a description of procedures and methodologies used in the identification of these countries and the international support measures available to them. It builds upon and updates the previous edition, published in 2018. The Handbook aims at providing comprehensive and up-to-date information on the LDC category. The publication is intended for use by government officials, policymakers, researchers and others interested in the LDC category. The Handbook is organized as follows: chapter I provides a detailed description of the procedures for inclusion in and graduation from the category. Chapter II presents an overview of the international support measures accorded specifically to LDCs, including measures related to trade, development assistance and support for participation in international forums. Specific attention is given to the impact of graduation on these support measures. Lastly, chapter III provides a detailed explanation of the LDC criteria, including composition, methodologies and data sources. In addition, the chapter presents specific examples of the application of the criteria, based on the Committee for Development Policy (CDP) 2021 triennial review of the list of LDCs. As measures of support, provisions, procedures and methodologies evolve over time, the information contained in the present Handbook will be updated on a regular basis to reflect relevant developments, including the outcome of the triennial reviews of the list of the least developed countries. Updates will be posted at www.un.org/ldcportal/. Up-to-date detailed information, including statistical data on the LDC category, is also available on the CDP website at http://cdp.un.org.