Land and freshwater molluscs are a moderately large group of animals with just over 200 species known at present from Britain and Ireland. Many of the species are particularly sensitive to changes in land use and hydrology, and many are sensitive to pollution or disturbance. Therefore, they are one of the most useful groups of animals for assessing biodiversity and determining change in the quality of a particular habitat. There is a long history of their study, and detailed records extend back over 130 years. This not only allows change to be detected, but with repeated and ongoing recording, the changes may be accepted as real. This chapter gives an overview of the non-marine mollusc fauna of Britain and Ireland, the changes in the fauna over the last half century and gives examples of species whose range has reduced and the reasons why this decline has occurred. It is estimated that nearly 40% of the native species in Britain are in decline. Many freshwater snails have undergone major contractions in geographical range and the number of known sites. Large species of freshwater mussels have been affected both by pollution and the effects of invasive alien species. Very few native land or freshwater species are showing any signs of recovery and the prospects are generally poor. In contrast, the number of introduced species continues to increase and the majority of them are spreading rapidly.