The paper offers a distinctive reading of Popper’s work, suggesting that his Logic of Scientific Discovery (LScD) might be re-interpreted in the light of his Open Society. Indeed, Popper can be interpreted as criticising certain aspects of his first book, and as a result improving upon them, in his second. It suggests translating what Popper says about ‘conventions’ into his later vocabulary of ‘social institutions’. Looking back, I believe that Popper never intended the language of conventions and decisions to be read individualistically. I remain unsure whether Popper was himself quite as clear about this as he could have been. My reading makes Popper a pioneer in the sociology of science. Scientific institutions are arenas of political power; but Popper did not discuss the structure and inter-relations of the social institutions of science, or offer a politics of science in the context of his methodology. What is missing from the skeletal sociology of LScD is the politics. We could put it in Popperian terms this way: scientific institutions are both open and closed. They are closed, firmly, to the inexpert, to the non-members; supposedly they are open to the qualified, provided the prerogatives of seniority and leadership are acknowledged. Despite these shortcomings, Popper’s critical and rational approach and his insistence on openness and intellectual honesty are still important today.