Despite their often temporary nature, intermittent rivers and ephemeral streams (IRES) provide important habitat, resource advantages, and nurseries for a range of fish species throughout the world. Fishes have adapted to the variable nature of these environments. A small subset of species using IRES has evolved specific traits such as air-breathing that make living in extreme habitats possible, but the majority relies on opportunistic dispersal to colonize such areas. Fish existence in IRES is precarious but when conditions are favorable, fishes play major roles in ecosystem functioning (e.g., trophic links in food webs). Habitat fragmentation of IRES by water extraction, river regulation, and climate change is the biggest threat to their ecological integrity and has major implications for the distribution and abundance of fishes in IRES.