Tagging of fi sh has been carried out at least as long ago as the 17th century when, in ‘The Compleat Angler’ (fi rst published in 1653), Izaak Walton reported the attachment of ribbon tags to the tail of juvenile Atlantic salmon Salmo salar to determine their movements (Walton and Cotton 1921; Lucas and Baras 2000). Since then, systematic tagging of fi sh for scientifi c purposes has been conducted for more than a century using natural marks or synthetic passive marks and tags, whereas the development of electronic tags arose in the 1950’s with the fi rst study of Trefethen (1956) using underwater telemetry. Methods to study the migratory behaviour of fi sh can be divided in two categories: capture-dependent (based on sampling marked or unmarked fi sh) and captureindependent methods, such as visual or video observation, resistivity fi sh counters or hydroacoustics (Lucas and Baras 2000). In this chapter we focus only on capturedependent methods using marked fi sh.