For 17 years, Willis was an essential component of Frank Zappa's music: as lead singer and rhythm guitarist, joining Zappa on six world tours, and appearing on no less than 30 of Zappa's official releases (most recently the posthumous Trance-Fusion and Buffalo) until Zappa's death from prostate cancer in 1993. Since that time he as toured, performed and/or recorded with: The Muffin Men, Banned From Utopia, Project/Object, Ed Palermo, The Central Scrutinizer, Ossi Duri, Cuccurullo Brillo Brullo and others. Since its inception, Willis has been a prominent guest at the Zappanale festival, and is scheduled to take part in the 2007 edition of this festival in Bad Doberan, Germany. Also in 2007, Willis has re-recorded and re-released a couple of his earlier solo albums: Should'a Gone Before I Left (1988) and Dirty Pictures (1998).