The theorem that the combined operation of changing charge conjugation C, parity P, and time reversal T, denoted CPT, is a fundamental symmetry of relativistic quantum field theory. No violation of the CPT theorem is known experimentally. When C, P, and T (or any two of them) are violated, the principles of relativistic quantum field theory are not affected; however, violation of CPT invariance would drastically alter the fundamentals of relativistic quantum field theory. It is not known whether superstring theory obeys a version of the CPT theorem.