Mountain environments are extremely important to the world’s water resources, weather systems and populations. This Special Issue of Hydrological Processes, edited by Carmen de Jong, Damian Lawler and Richard Essery, contains 12 key research papers from leading international research groups on the important theme of ‘Mountain Hydroclimatology and Snow Seasonality’. The Special Issue is largely based on mountain and snow hydrology sessions which the Editors organised at successive European Geosciences Union (EGU) General Assembly meetings in Vienna in 2005 and 2006, and these are ably supported by some Invited Papers. These forward-looking EGU sessions under the banner of ‘Mountain Hydrology and Climatology: present state and future scenarios’ aimed to (a) identify key research gaps and (b) set out and discuss the most promising approaches, methodologies, datasets, relationships and conclusions to address these research lacunae. The introductory paper by the Guest Editors provides an overview of the hydrological importance of mountain regions, outlines some recent developments in the field and then introduces the following 11 papers which comprise this timely Special Issue.