How do atomic clocks keep time so accurately?

Atomic clocks use the regularity of atoms, especially the microwave transition of cesium-133, to keep time with extraordinary precision. The official SI second is defined by 9,192,631,770 cycles of the radiation associated with a cesium atom transition. Because atoms are so stable, atomic clocks can lose or gain less than a second over millions of years. They are essential for GPS, telecommunications, and scientific measurements. Modern optical atomic clocks are even more precise than traditional cesium clocks, though the cesium definition still anchors the second.