This philosophical treatise on the foundations of semantics represents a systematic effort to clarify, deepen, and defend the classical doctrine that words are conventional signs of mental states; principally, thoughts and ideas; and that meaning consists in their expression. This expression theory of meaning is developed by carrying out the Gricean program, explaining the significance of words to have meaning in terms of speaker meaning, and for a speaker to mean something in terms of intention. Grice's own formulations are rejected, and alternatives are developed.