Brian Simon (1915-2002), a Marxist historian and educationist in Britain, was a leading pioneer in the comprehensive education movement. Although Simon's great contribution to this movement has been recognised by historians, the development of Simon's ideas and his actions in this movement have hardly been examined in great depth by previous literature. Hence, this article explores Simon's ideals of the comprehensive school and his vigorous involvement in the movement, especially from the immediate post-war years to the issue of Circular 10/65. Furthermore, as Simon was a member of the Communist Party and greatly influenced by Marxism, this article also focuses on how Simon's ideals and actions were underlined by his Marxist critique of intelligence testing and the concept of inherited 'intelligence' itself.