This chapter is an examination of Britten’s engagement with progressive musical and aesthetic thought. As a successful and popular composer, Britten is rarely identified as an ‘avant-garde’ artist, yet his career took note of progressive developments from 1930s neoclassicism to 1970s minimalism. For mid-century critics, Britten was a cosmpolitan figure; more recently, his commitment to tonality argues a ‘reactive modernism’, in dialogue with tradition. Britten’s relations to avant-garde thought involve successive historical contexts. In the 1930s, he sought to study with Berg, wrote experimental film soundtracks, and explored neoclassical parody, without abandoning key tonality. In the 1940s, Britten’s music developed greater metric complexity. Britten’s 1950s catalogue increasingly explores a personal twelve-tone thematic idiom, along with non-European percussion sonorities inspired by renewed encounters with Balinese gamelan. Criticising avant-garde ‘complication’ in the 1960s, Britten tempered public scepticism with personal support for British avant-gardists.