The CPT theorem is a fundamental principle in quantum field theory that asserts any Lorentz-invariant local quantum field theory with a Hermitian Hamiltonian must be invariant under the combined transformations of charge conjugation (C), parity transformation (P), and time reversal (T). This theorem implies that the laws of physics remain unchanged when particles are replaced with antiparticles, spatial coordinates are inverted, and the direction of time is reversed simultaneously.