The London Sinfonietta’s reputation as a contemporary music ensemble comes from the continuing commitment by its players and artistic directors to its quality of performance. The ensemble also developed a sense of its musical identity by generating a repertoire of commissioned works. This article looks at some of the ways that the Sinfonietta defined its musical ideology and has functioned as a performance-centred art world in which composers are invited to collaborate but are not there by right. It examines the Sinfonietta’s strategies for audience building, the ensemble’s contribution to the development of London’s concert life since its foundation in 1968, and looks at the ways that it has sought to renew itself in response to the different funding and aesthetic environments it has encountered in its history. The Sinfonietta’s venture into Broadway repertoire is discussed in terms of social identity. The article argues, after Jürgen Habermas and T. W. C. Blanning, that one of the most significant contributions of the Sinfonietta has been its continued commitment to the concert event as a forum for critical engagement and cultural debate.