Unionoida (naiads) are characterized by larvae which have to pass through a parasitic stage on a host fish. Some of these host-parasite systems are unique, since the generation time of the parasite exceeds that of its host by a factor of ten. There is tremendous life history variation. With a life span of more than 200 years, some naiad species belong to the longest-lived invertebrates, some are highly host-specific, some are extremely fertile, some produce very peculiar larvae, and some occur at very high population densities. This volume describes and explains the characteristics and life histories of the naiads, the interactions with their hosts, and their evolution. It elucidates the manifold implications of their presence or absence in a lake or stream. Further, aspects of nature conservation are covered, as many naiad species are seriously threatened. Some have been used successfully as sensitive pollutant indicators in habitat monitoring and as archives for environmental changes.