From a hydrological perspective, Chile is a country of extremes. Its rivers and aquifers exist in a wide array of geographical settings, and are characterized by presenting regimes of extraordinary variability in space and time. From the driest desert on Earth to Patagonia, communities and ecosystems have adapted to endure periods of persisting drought, relentless precipitation, as well as favorable conditions for the emergence of a burgeoning agricultural sector. With mounting evidence of global change processes, it is still unclear how these regimes will evolve in the future, and what are the challenges that water managers will need to face in order to balance increasing water demand and the need for water security. This chapter describes the major hydrological regimes associated with water resource relevant regions in the country, highlighting the hydrological processes, variability and uncertainties pertaining to water resource management. A global assessment of the state of hydrologic knowledge, data availability and future directions for research and management are provided.