An axiomatic theory of truth is a deductive theory of truth as a primitive undefined predicate. Because of the liar and other paradoxes, the axioms and rules have to be chosen carefully in order to avoid inconsistency. Many axiom systems for the truth predicate have been discussed in the literature and their respective properties been analysed. Several philosophers, including many deflationists, have endorsed axiomatic theories of truth in their accounts of truth. The logical properties of the formal theories are relevant to various philosophical questions, such as questions about the ontological status of properties, Gödel’s theorems, truth-theoretic deflationism, eliminability of semantic notions and the theory of meaning.