"Baba O' Riley" began as part of Townshend's Lifehouse project, which is a film script he wrote. It was one of the first times a synthesizer was used to form the rhythm of a song. The synthesizer part was played from tape at their concerts because it was too difficult to reproduce live. In concert, Roger Daltrey almost always plays the violin jig at the end of the song on harmonica.
This is the great version of it from the excellent documentary The Kids Are All Right (1979). Blognor Regis says: "The Kids Are Alright gig was at Shepperton Studios on the 25th May, 1978. It was their only gig of the year. Moon the Loon would die on 7th of September":