The majority of the twelve species of diadromous fish found in mainland France are considered as vulnerable or endangered. To understand their ecology, numerous studies have been carried out to collect individual data, notably using a non-lethal method of analysis for fish: scale sampling. This book highlights the possible uses of scales for monitoring and observing seven of the twelve French diadromous species: Atlantic salmon, sea trout, the three species of shad, mullet and smelt. The authors extend the analysis of scales to other types of data interpretation, from scalimetry (scale- reading and back-calculation)—the most common method for estimating the age and some traits of the life of fish—to isotopic and microchemical studies, genetic characterization, the recognition of species, biological forms, fish of domestic origin, and stock discrimination. These uses are illustrated with examples taken mainly from the salmonid and alosine families. This practical guide is intended for professionals in research and Science-Policy-Interface-offices, researchers and students involved in aquatic environment research, as well as diadromous fish associations and fishing and environmental protection federations.