Oscillations of neutral B mesons systems

The oscillation phenomenon in the neutral B mesons systems is now well established. The motivations and principles of the measurements are given; then the most recent results from the LEP experiments, the CDF collaboration at Fermilab and the SLD collaboration at SLAC are reviewed. The present world average of the $\bd$ meson oscillation frequency is $\dmd = 0.471 \pm 0.016 \ps$ and the lower limit on the $\bs$ oscillation frequency is $\dms > 12.4 \ps at 95% CL$.