This chapter looks at Len Lye’s filmmaking throughout the 1930s, and especially his relations with the General Post Office (GPO) Film Unit. While discussing the nature and extent of his involvement in the British documentary movement, the chapter also situates Lye’s work within a wider visual and film culture. In particular, it examines how his filmmaking combined animation with found-footage and live-action documentary techniques. Lye’s GPO films – like those of Norman McLaren, Lotte Reiniger, and Humphrey Jennings – complicate commonplace assumptions about the documentary movement of the inter-war era, and its relations to Modernism. There was an anarchic quality to Lye’s career, and while he was by no means the only experimental filmmaker associated with the GPO unit, the extent of his ability to innovate – in spite of meagre to non-existent production budgets – with colour processes, musical compositions, and stencils, exemplifies a concept of filmmaking that successfully blurs the boundaries between documentary and other visual arts.