Global Harmful Algal Bloom Status Report 2021 The most frequently asked questions about harmful algal blooms are if they are increasing and expanding worldwide, and what are the mechanisms behind this perceived escalation. These questions have been addressed in several review papers concerning HAB trends at various scales, where evidences of expansion, intensification and increased impacts of harmful algal blooms have been gathered from a selection of examples that have gained high prominence in the scientific world and in society. Eutrophication, human- mediated introduction of alien harmful species, climatic variability, and aquaculture have all been mentioned as possible causes of HAB trends at various spatial and temporal scales. The lack of a synthesis of the relevant data has hampered a sound global assessment of the present status of phenomena related to harmful algae. This Global HAB Status Report for the first time compiles an overview of Harmful Algal Bloom events and their societal impacts; providing a worldwide appraisal of the occurrence of toxin-producing microalgae; aimed towards the long term goal of assessing the status and probability of change in HAB frequencies, intensities, and range resulting from environmental changes at the local and global scale. This initiative was launched by the IOC Intergovernmental Panel on HABs (IOC/ IPHAB), and has been pursued with the support of the Government of Flanders and hosted within the IOC International Oceanographic Data and Information Exchange Programme (IODE) in partnership with ICES, PICES and IAEA.