This chapter explores the fundamental categories of Locke's ontology. It argues that these are substance and mode. He conceives of substances as ontologically independent things or stuffs. His notion of mode is non-standard, including events as well as features of substances. When the idea of a feature contains different kinds of ideas as parts, Locke calls this the idea of a mixed mode. He sometimes seems to deny the reality of mixed modes, but this chapter makes the case that he does not really mean to do so. However, it does argue that he means to deny the existence of relations.