Ian McCarthy was one of Australia's outstanding theoretical physicists. He was born in country South Australia and, after a PhD at the University of Adelaide, and periods of work in the UK and USA, he returned to South Australia where for several decades he led an outstanding research program at Flinders University. Ian's career had two major stages. In the first, he made major contributions to nuclear reaction theory, including very important insights into the physical consequences of the optical model and state-of-the-art calculations of proton knock-out from nuclei. In the second phase, he imported the concept of the knock-out reaction to atomic, molecular and solid state physics. Using the (e,2e) reaction, for which he and his colleagues developed the theoretical framework, his group made major contributions in these areas.