This paper traces the development of Popper's thinking on propensities from his writings on probability during the 1950s through to his 1990 bookA World of Propensities. It is argued that there are significant commonalities between the views expressed in this (his last) book and the metaphysics and methodological implications of Critical Realism. The paper concludes with some consequences for Popperian falsificationism, situational determinism and Popper's later view that propensities may be interpreted as causes.