Volcanology has made great advances in the last 3 decades, becoming a modern interdisciplinary science that has learned how to quantify volcanic processes, their associated hazards and resources, as well as the impact of volcanic activity on society and the environment. Numerical models are developed in order to better understand volcanic processes and aid in hazard prediction. At the same time, an accurate forecasting of future volcanic events requires detailed understanding of past trends. Because models can describe only a simplified version of complex natural processes, geologic and field data are critical for constraining model input, necessary for model validation, and will ultimately improve model accuracy. The aim of this special issue is to present studies in which traditional field-based methods are conceived and used for constraining, improving, and validating numerical models describing volcanic processes. We collect contributions describing field data and the role of volcano geology in model validation for a wide range of volcanic phenomena, including tephra fall, pyroclastic density currents, and lava flows. The contributions listed below arise out of both an IAVCEI COV10 session in 2018 and an open call for contributions. The volume is under aegis of IAVCEI Volcano Geology commission.