Salmonids like rainbow trout Oncorhynchus mykiss, brook trout Salvelinus fontinalis and brown trout Salmo trutta are potent invaders in various regions throughout the world, but non-native rainbow trout have become established in only a few locations in the U.K. to date. Salmonid invasions in other regions are often driven by repeated authorized introductions of large numbers of fish, escapees from fish culture and illegal introductions by anglers. Their spread can be rapid within catchments via long-distance movements. Currently, large numbers of all-female diploid rainbow trout are released in the U.K., the eggs of which could be fertilized by relatively few mature males, thereby increasing the risk of invasion. If reproducing fish began to spread, especially by anadromy, then eradication could be difficult due to angler interest and logistical constraints. Establishment of non-native salmonids can be limited by abiotic factors such as flow and temperature regimes, and biotic resistance from competition, predation and parasites or diseases, which often affect early life stages most strongly. Moreover, these factors can also interact with the zoogeography and evolutionary history of the native and introduced salmonids to make explanation and prediction complex. Although flow and temperature regimes in the U.K. are broadly suitable for rainbow trout, and biotic resistance by brown trout has been overcome in other regions, resistance from native diseases and angling mortality appear to be plausible hypotheses for lack of rainbow trout invasion in the U.K. Invading salmonids can have both direct effects on native salmonids and other fishes via biotic interactions, and indirect effects by fragmenting their habitat and isolating native populations. In addition, invaders can have indirect effects on entire aquatic food webs (e.g. invertebrates and algae) that cascade throughout ecosystems, or even cross boundaries into adjacent ecosystems. Evidence from other regions indicates that rainbow trout could have negative effects on brown trout or Atlantic salmon Salmo salar in U.K. waters, but this probably depends on the context set by other factors. Overall, primary factors to consider in any risk analysis of rainbow trout invasion in the U.K. include the decline of native salmonid populations due to degradation or overexploitation, the evolutionary history of newly introduced rainbow trout stocks, local or global influences that change aquatic environments and reduce biotic resistance by native parasites or diseases, and rapid evolution of local adaptations by new or marginally established populations of rainbow trout that makes them more invasive.