Recent years have seen significant advances in our capacity to track volcanic ash clouds remotely (from space, or from the ground), and in the modelling of volcanic ash plumes. These technical advances have significant potential to assist the reconstruction of past eruptions, to improve real-time operational forecasting during volcanic eruptions, and to develop further the modelling of hazards associated with tephra dispersal and fallout. But many challenges remain: do we know enough about the optical properties of 'ash' for satellite detection and measurement of volcanic ash plumes? Are the physical properties of ash (e.g., density) characterised in sufficient detail for tephra dispersal modelling, and how can models help us to reconstruct the key parameters that characterise ancient tephra fall deposits? What are typical uncertainties related with the different approaches and techniques? And what are the 'gaps' between the approaches: the coarse tephra that cannot be 'seen' by satellite, or the fine ash that is absent from the geological record? This session invites contributions across the full spectrum of this topic, with the aim of stimulating further discussion and future work.