"Firth of Fifth" is a progressive rock instrumental piece by the British band Genesis. It appears on their 1973 album, Selling England by the Pound. The song is known for its intricate musical structure, featuring a blend of piano, guitar, and flute, showcasing the band's technical prowess. The title refers to a geographical feature, as "firth" denotes an estuary or inlet. The composition is divided into several sections, each highlighting different instruments and themes. It has become a fan favorite and is often praised for its melodic complexity and lyrical depth, despite being primarily instrumental.