Snowpack density is one of the most difficult hydroclimatic variables to be monitored in mountain environments. Snow pillows are a commonly used tool for this purpose. Despite they have been widely used in North America, Scandinavia, and several mountains areas, they had not been installed yet in any Spanish mountain system. In 2005, a snow pillow was installed in Izas Experimental basin (Central Pyrenees). Since then, it has provided information on the evolution of density and snow water equivalent (SWE) for a three years period. This paper reviews the history of the use of snow pillows, specifics of its utilization and considerations to have into account when they are installed. Moreover, results obtained during the three monitored years are presented. Several uncertainty sources are detected, and the usefulness for understanding the hydrological behaviour of high mountain environments is highlighted.