This paper deals with the evolution of the issue of monopoly and large firms in Joan Robinson’s writings. This issue reveals her growing detachment from the hypotheses and methods used to explain economic phenomena and constituting the basis of liberal orthodoxy. This methodological rupture (after 1933) led her to the view that monopoly represents a logical and natural end to the competitive struggle. Robinson’s thought is also constructive, and this paper highlights her place in the development of theories of the firm and in the current relevance of her analysis of the power of large firms in circumstances of monopoly or oligopoly.