The 18th International Conference on Harmful Algae (ICHA) was held at La Cité des Congrès, Nantes, France, 21-26 October 2018. Philipp Hess, editor of the proceedings, chaired the local organizing committee with the support of the French GdR PHYCOTOX and GIS Cyano societies. The theme of the conference “From Ecosystems to Socio-ecosystems” was chosen to stress the need to integrate natural sciences with social sciences to increase the impact of HAB science on society. This edition has enjoyed record attendance (709 participants from 64 countries) and scientific contributions (613 abstracts, with 255 oral presentations and 358 posters, 45 of which were also presented as ignite talks). It consisted in nine plenary lectures and 246 other lectures subdivided into 21 themes. Less than 10% of contributions to the conference resulted in submissions to the proceedings. However, these 47 papers cover 17 of the 21 topics of the conference.