In many areas of the United States (US), application of deicing salts is a major contributor to elevated and increasing chloride concentrations and specific conductance. Elevated chloride concentrations can negatively affect aquatic ecosystems and drinking water quality. Understanding the factors that control chloride (and sodium) concentrations is essential to inform efforts to improve deicing salt application practices and reduce impacts on ecosystems and drinking water quality. Short-term variations in chloride concentrations over days to seasons are most influenced by direct inputs to streams and other surface water, frequently entering via stormwater drains. As a result, in many places chloride concentrations are higher in the winter. In-stream sensors measuring on the time scale of minutes often provide useful information on concentrations, especially during the winter. Long-term variations in chloride concentrations over years to decades are caused by indirect inputs to surface water, typically entering via groundwater inputs at baseflow. Over the long-term, concentrations have been increasing for years to decades in many locations. An unintended but related consequence of stormwater management is increasing chloride concentrations in groundwater, which causes stream concentrations to increase over the long-term. A key wrinkle in understanding the susceptibility of aquatic communities is that the negative effects of chloride are connected to both background conditions (surface waters with low versus high dissolved solids) and to the mix of ions present in surface water.