This document discusses the use of unmanned aerial systems (UAS) for hydrological monitoring. It describes how UAS can be used to monitor vegetation status, streamflow, flooded areas, and river morphology from the plot scale to the river basin scale. It outlines the COST Action HARMONIOUS, which aims to promote UAS monitoring strategies and establish harmonized practices through a global network of over 200 researchers from 36 countries. The document provides examples of UAS applications for monitoring vegetation stress, soil moisture, and streamflow and discusses efforts to standardize UAS data processing and analysis methods across different environments.