In this talk I review the status of the theory prediction of the parameter eK. eK describes CP violation in the neutral kaon system and is one of the most sensitive probes of new physics. Large uncertainties related to the charm-quark contribution to eK have so far prevented a reliable standard-model prediction. I will show that CKM unitarity enforces a unique form of the |∆S = 2| weak effective Lagrangian in which the short-distance theory uncertainty of the imaginary part is dramatically reduced. The uncertainty related to the charm-quark contribution is now at the percent level.