Recent studies have documented the characteristics and importance of narrow regions of strong meridional water vapor transport, which are contained within extratropical cyclone warm sectors. These regions have recently been referred to as “atmospheric rivers,” and are the subject of this presentation. They play key roles in both the global water cycle and in determining the day-to-day location and intensity of precipitation (i.e., weather). From a climate perspective, Zhu and Newell (1998) showed that >90% of the meridional water vapor transport at midlatitudes takes place in atmospheric rivers constituting <10% of the earth's circumference at those latitudes. In terms of weather, these features profoundly influence precipitation events and extremes. For example, roughly half of the rainfall observed in the coastal mountains of California over 4 winters occurred during the landfall of atmospheric rivers, as did 87% of the hours that had rainfall >10 mm/h. From a hydrologic perspective, Ralph et al. (2006) showed a clear linkage to flooding through a study of the Russian River where all 7 floods since 1997 (i.e., when the experimental observations needed to detect atmospheric rivers first became available) occurred when atmospheric river conditions were present and caused heavy rainfall through orographic precipitation. A related 8-year study covering Western North American found that roughly twice the normal precipitation fell when winter storms exhibited atmospheric river attributes, and that winter snowpack experienced greater than normal gains during such events. Descriptions of the current physical understanding of atmospheric rivers and their hydrometeorological impacts will be provided, including a synopsis of their role in linking weather and climate in the water cycle. An overview of gaps in the science will be included, as will a summary of the capabilities and limitations of our current observational monitoring capabilities. Emerging alternatives to address these gaps will then be noted.