John William 'Jo' Callis is a British musician and songwriter whose contribution was vital in transitioning the English synth-pop band The Human League from an avant-garde electronic group into international pop superstars. Originally a punk rock guitarist for the Rezillos, Callis joined The Human League in 1981 just as they were restructuring their sound. He stayed with the group through their most commercially successful "imperial phase" until his departure in 1984.