  From the nineteenth century onwards, when serious versions of scientific atomism first emerged, the philosophical relevance of a history of atomism becomes epistemological rather than metaphysical. Since atoms lie far beyond the domain of observation, should hypotheses concerning them form part of empirical science? There were certainly philosophers and scientists of the nineteenth century who answered that question in the negative. Contemporary philosophers differ over the question of whether the debate was essentially a scientific one or a philosophical one. Was there a case to oppose atomism on the grounds that it was unfruitful or lacking in adequate experimental support, or did such a case stem from some general epistemological thesis, perhaps some brand of positivism, that ruled out of court any attempt to explain observable phenomena by invoking unobservable atoms? Many contemporary philosophers see the ultimate triumph of atomism as a victory for realism over positivism. Such claims are historical as well as philosophical, so it is important to get the history straight when evaluating them. In this respect the philosophical literature has yet to catch up with recent advances in the history of nineteenth-century chemistry. This entry gives an account of the key developments in atomism from the seventeenth century until the time, early in the twentieth century, when the existence of atoms ceased to be a contentious issue. The focus is on the epistemological status of the various versions, and on the relationship between science and philosophy.