Brackish water can be found wherever sea water is diluted by freshwater to form water of intermediate salinity. The ephemeral habitats of estuaries, coastal lagoons and inland seas are rich in fauna, but are threatened by rising sea levels, drainage and reclamation of low lying coastal land, storms and sea defences. This is the first identification guide to the fauna of these ecologically important areas. The ecology of these animals and their habitats are examined and reviewed. The keys are designed to be used with living material, and illustrations are provided for all species likely to be found in brackish-water habitats in northwestern Europe, making identification easy. Fieldworkers, naturalists and students interested in these fragile and ephemeral habitats will find this an indispensable reference.