Grafton Elliot Smith rose from a colonial Australian background to dizzying heights in the British scientific establishment. He became a world authority on neuroanatomy and human prehistory, holding chairs at Cairo, Manchester and University College, London. He was best known publicly for his challenging theory of cultural diffusion, crossing the boundaries of anthropology, archaeology and history, stemming from his expert knowledge of evolution. Most controversy raged about his "Egyptian" theory, which placed ancient Egypt as the dynamic source from which major elements of civilisation were spread by the migration of peoples and mores. This vision stemmed from his ground-breaking dissection of thousands of mummies in Egypt during the great excavations of the 1900s. His speculations, made in association with thinkers such as W H R Rivers and W J Perry, bore fruit in a spate of publications that sparked global debate, arousing particular anger from American ethnologists opposed to ideas of foreign influence upon Mesoamerican cultures. Elliot Smith's ideas were regarded at the time as authentic, if problematic, approaches to important issues in human history. They were subsequently to be caricatured or ignored in anthropological and archaeological disciplines that had moved on to other paradigms. Paul Crook shows how his ideas were developed in the context of his life and times, examining the debates they aroused, his attempts to incorporate anthropology within a broader interdisciplinary school under his leadership in London, and his opposition to Nazi race theory in the 1930s. There has been no full-scale biography of Elliot Smith and little of substance analysing his works. Despite shortcomings, his theory and reputation deserve rehabilitation. An Afterword brings general readers up to date about the whole "diffusion" debate.