The alteration of river flow regimes by dams, weirs, and water extraction is a key impact of humans on freshwater ecosystems and has occurred in rivers across all inhabited continents worldwide. In this chapter, we provide a brief summary of the expansive literature on the ecological consequences of flow regime change. We focus on links between alteration of specific components of the river flow regime and the typical patterns of ecological response. Flow regime change is evident in terms of altered baseflow(s), reduced flooding magnitude and frequency, reduced floodplain inundation, and altered flow variability" however, the extent to which these hydrological components are altered depends on the driver of hydrological change. Effects of flow regime change are evident for multiple organism groups (i.e., plants, amphibians, invertebrates, and fish), ecological processes and in-channel, riparian, and floodplain environments. Altered richness, abundance and density, composition, and altered process rates are reported as responses to changes across different hydrological components. However, the characteristics of ecological responses vary in response to change across hydrological components and are also strongly determined by local factors such as hydroclimatic region, biological traits of organisms, and how flow regime change is manifest in terms of stream hydraulics. A strong conceptual understanding of the ecological effects of flow regime change is essential for predicting, monitoring, and evaluating the responses of environmental water releases, and how environmental water can be delivered to enhance ecological outcomes.