The accumulation of knowledge concerning the character and transformations of substances from ancient times constitutes progress in chemistry, which has accelerated enormously since the end of the seventeenth century. The chapter focuses on some themes in the development of theorising and conceptual clarification at the macroscopic and microscopic levels during the nineteenth and twentieth centuries. This covers the general understanding of substances in relation to phase and the general notion of a mixture, the distinction between daltonides and berthollides, and issues connected with the notion of molecular structure. Together with the enormous body of factual material accumulated particularly over the last two centuries, this is progress in chemistry. The epistemological approach seems to give the best account of it amongst philosophical views on progress currently on offer.