An inspiring speaker and brilliantly sophisticated theorist, Michael Kidron was a leading figure in the International Socialist tradition from the 1950s until his death in 2003. Never satisfied with merely restating the assumed tenets of Marxism, Kidron insisted that theory must evolve alongside a changing world - an iconoclastic orientation which led him to clash with others on the left, including the British Communist Party and, later, the Socialist Workers Party itself under the leadership of Kidron's long-time comrade Tony Cliff.