In many of its claims, Bolzano’s metaphysics appears to belong in the eighteenth century: he elaborates a monadology and an account of the creation which, despite significant differences, resemble those of Leibniz and his successors in many particulars. As in mathematics, he treated the work of his predecessors with respect, seeking foundations for what he took to be solid and modifying where he thought necessary. As in mathematics, too, his search for foundations often produced radically new conceptions. Perhaps the most important of these are found in his theory of collections, with its anticipations of set theory and classical mereology. This chapter gives a survey of Bolzano’s work in metaphysics, covering basic concepts of his ontology (object, attribute, property, relation, determination, collection, substance, adherence, etc.), his account of space and time, and his atom-theory (a variant of Leibniz’s monadology), as well as his conceptions of necessity and possibility. (148 words)