Unsolved Problems in Hydrology have been identified through a community consultation and prioritization process, and formalised by Blöschl et al., 2019. Twenty-three Unsolved Problems in Hydrology (UPH) – a community perspective. Hydrological Sciences Journal, 64, 10, 1141-1158, https://doi.org/10.1080/02626667.2019.1620507. Several thematic sessions of the 2022 Assembly will articulate with some of these UPHs. This overarching session will welcome communications about the overall epistemic challenges and progress.