John Williams is best known for such large-scale symphonic works as Star Wars, Superman, the Indiana Jones series and the likes. Yet, his musical roots are also (and strongly) in jazz. In his formative years he had the opportunity to be in touch with some of the finest performers. In the 1950s, while studying 'legitimate' piano music at the Juilliard School in New York, he tickled the ivories in the city's jazz clubs. In the early 1960s, while working in Hollywood as a pianist and an orchestrator, he arranged albums for Mahalia Jackson and Vic Damone. When Williams firmly established himself as a film composer the mid-1960s, he brought in his jazz background as a perhaps less noticeable but yet fundamental component of his style. After a quick introduction about the relation between cinema and jazz in Hollywood history in order to contextualise Williams's film career, the chapter focusses on the Williams film scores in which jazz episodes emerge from beneath the otherwise symphonic texture – for example, 'Cantina Band' in Star Wars (1977), 'Swing, Swing, Swing' in 1941 (1979), 'The Knight Bus' in Harry Potter and the Prisoner of Azkaban (2004) – and on film scores in which jazz is central to the film's narrative: for example, Catch Me if You Can (2002) and The Terminal (2004).