This chapter discusses the friendship and mutual interests of Isaac Newton and the English philosopher John Locke. It is structured around their common interests in chymistry, theology, and natural philosophy. Newton and Locke were both friends of the leading chymist Robert Boyle and much of their correspondence regarding chymistry revolves around him. Their discussions in theology focused on (what we now call) textual criticism of biblical texts that had implications for the doctrine of the Trinity. It is their common interest in natural philosophy, however, that is the most interesting and revealing facet of their relationship. This began with Locke’s review of Newton’s Principia in 1688 and is in evidence in Locke’s Essay and other writings as he came to terms with the Newtonian achievement. Locke’s philosophy, in turn, seems to have influenced Newton, though there is only meagre evidence for this in Newton’s writings.