The K L - K S mass difference

We review the status of the RBC-UKQCD collaborations' computations of the K L -K S mass difference. After a brief discussion of the theoretical framework which had been developed previously by the collaboration, we describe our latest computation, performed at physical quark masses, and present our preliminary result m K L - m K S = (5.5 ± 1.70) × 10 -12 MeV.