The freshwater fish fauna of Britain and Ireland is an impoverished one compared to much of the rest of Europe, due to the effect of the last Ice Age some 10,000 years ago and to the more recent separation of Britain from the continental land mass. For the same reasons there is a significant difference from north to south in local fish faunas. The major pattern of change in freshwater fish populations in these islands over the last 200 years has been a decline in native, especially northern, species and a parallel increase in non-native and some southern species. Some native species like Burbot and Houting are now extinct, whilst others, for example Vendace, Smelt, Allis and Twaite Shad, and even European Eel, are declining. In contrast, southern species like Ruffe, Dace and Rudd are extending their range into northern catchments, thanks to introductions by coarse anglers, and increasing numbers of non-native species, for example Sunbleak, Topmouth Gudgeon and Black Bullhead, are being successfully introduced. Climate change appears to be enhancing these changes. Of the 57 freshwater fish species known from Britain and Ireland, 14 are alien. Only in the last few decades has there been any attempt to reverse these changes through legislation, habitat restoration, catchment management and conservation management of rare species. It is emphasised that habitat protection and restoration are the principal means through which successful sustainable fish conservation and restoration of biodiversity will be achieved.