In this contribution I examine Klein's 1930 paper ‘The importance of symbol-formation in the development of the ego’, and try to show that it is one of her most important and seminal papers. In the paper she boldly states that the capacity for symbol formation is crucial in the development of the ego. She links it with the development of object relationships, and emphasizes the role of aggression and anxiety as well as the libido. It foreshadows her concept of projective identification. The paper opened a new area of investigation: it was seminal in provoking new developments and in furthering understanding of psychotic disturbances in the development of thinking and sublimation.