The defining feature of all intermittent rivers and ephemeral streams (hereafter, IRES) is that they cease flow at some time. Many IRES dry to isolated pools but flow often continues through the hyporheic sediments below the streambed. If dry conditions persist, hyporheic flows may also cease and the streambed dries completely. Consequently, the flow regimes (e.g., frequency, magnitude, duration, and timing of flow events) of IRES and the presence of water are typically more variable than in nearby equivalent-sized perennial rivers and streams. This highly variable flow regime, especially intermittence, has major implications for the physicochemistry, biota, ecological processes, and management of IRES. Flow regimes of IRES have been primarily characterized using data from gauging stations, supplemented by diverse methods such as wet-dry mapping, various forms of imagery, and modeling. Flow data are often summarized as hydrological metrics such as variance in frequency, duration, timing, and rate of onset of intermittence that have been used to classify flow regimes of many of the world’s rivers. Such classifications reveal that IRES are globally abundant and that intermittence is increasing across much of the world, largely owing to climatic drying and water abstraction.