The T-schema is the inductive definition that lies at the heart of any realisation of Alfred Tarski's semantic theory of truth, expressing the commutation of truth over logical operators.

T-schema are often expressed in natural language, but they readily admit formalisation in many-sorted predicate logic or modal logic; such a formalisation is called a T-theory. T-theories form the basis of much fundamental work in philosophical logic, where they are applied in several important controversies in analytic philosophy. They are also a fundamental intuition behind model theory; or to put it another way, model theory implements them.