This chapter examines William of Ockham’s theory of mental speech, focusing on aspects that have been claimed to give rise to the possibility of pernicious ambiguity. Against these claims, which center on the varieties of reference allowed by supposition theory, we argue that Ockham’s theory remains coherent despite marginal instances of ambiguity. We review two types of suppositional ambiguity: Type 1, in which referring terms can be interpreted as suppositing personally, materially, or simply; and Type 2, in which supposition is personal but can vary in tense or modality. Type 1 ambiguity is ruled out, except in extreme cases, by the fact that “changing” the supposition of a term requires a conscious, reflexive act of the speaker; Type 2 ambiguity, while more of a live possibility, is generally either absent or harmless. In neither case does ambiguity seriously compromise the function of mental speech as a vehicle of human cognition.