The freshwater pearl mussel Margaritifera margaritifera (L.) remains Critically Endangered in Europe and Endangered throughout its world range in spite of many years of conservation efforts. It has been legally protected since at least the 1990s in most countries it inhabits. It also benefits from international protection under the European Union Habitat's Directive, which has opened up over 30 years of funding to improve and ultimately restore its conservation status in Europe. This monograph brings together the research and conservation efforts to date that have been undertaken across most of its range, which has increased our understanding of the species' life history, interactions with host fish, its ecology and habitat requirements. Efforts to understand the catchment, water quality and flow changes needed to rehabilitate populations are ongoing. The juvenile stages are extremely sensitive, and the natural environment of M. margaritifera is oligotrophic. Many rivers have changed considerably since juvenile recruitment was last successful. However, adults are long lived and populations can recover if targeted catchment-level restoration is undertaken, providing benefits for climate action as well as mussels. This publication provides a synthesis to assist research and conservation management.