The THM Commission sponsored a scientific symposium at the Cities on Volcanoes 2018 meeting in Naples, Italy. Session S01.26 Volcanic ash: from monitoring to impacts. Conveners: Takahiro Miwa | National Res. Inst. for Earth Science and Disaster Resilience (NIED), Costanza Bonadonna | Université de Genève, Nobuo Geshi | National Institute of Advanced Industrial Science and Technology (AIST), Marco Pistolesi | University of Pisa – Earth Sciences Dept, Alexa Van Eaton | USGS Cascades Volcano Observatory, and Sara Barsotti | Icelandic Meteorological Office